Tosafos on Amud Aleph (“D-Eyn”) makes a distinction between what induces the halakhic state of impurity of Niddah, such as for purposes of maintaining ritual purity in the Temple era for sacrificial meats and foods, and the prohibition against sexual intercourse. One might think they are identical, but they are not, and sometimes one state will not necessarily imply the other. Rav Soloveitchik (Reshimos Shiurim, ibid) notes that the Kuzari (3:49) makes a similar distinction:

וּמַה שֶּׁאֶצְלֵנוּ הַיּוֹם מֵאִסּוּר שְׁכִיבַת הַנִּדָּה וְהַיּוֹלֶדֶת, אֵינוֹ מִפְּנֵי הַטֻּמְאָה אֲבָל הִיא מִצְוָה גְרִידָא מֵאֵת הַבּוֹרֵא, וְכֵן מַה שֶּׁאֶצְלֵנוּ מֵהַרְחָקַת הָאֲכִילָה עִמָּהּ וּלְהִשָּׁמֵר מִקּוּרְבָתָהּ, אֵין זֶה כִּי אִם מְנִיעוֹת וּסְיָגוֹת שֶׁלֹּא יִתְגַּלְגֵּל הָעִנְיָן לִשְׁכַּב עִמָּהּ.

The prohibition of cohabiting with a woman in her period has nothing to do with impurity, but is an independent divine law. The practice we observe to keep some distance is but a restriction and hedge to prevent becoming aroused and ending up violating the sexual prohibition of Niddah.

This Kuzari and Tosafos indicates that this is not merely a halakhic point, but a philosophical distinction in how we understand the menstrual taboo.  In other words, the sexual prohibition is stemming from a Torah law, thus the prohibition of Niddah as put forth by Kuzari is no different than not eating meat and milk or shaatnez, with no additional stigma added. As with many Torah laws, we can conjecture its meaning but not be conclusively sure. The Gemara (Niddah 31b) offers at least one dimension of this prohibition:

תניא היה ר"מ אומר מפני מה אמרה תורה נדה לשבעה מפני שרגיל בה וקץ בה אמרה תורה תהא טמאה שבעה ימים כדי שתהא חביבה על בעלה כשעת כניסתה לחופה

It is taught in a baraisa that Rabbi Meir would say: For what reason does the Torah say that a menstruating woman is prohibited from engaging in intercourse with her husband for seven days? It is because if a woman were permitted to engage in intercourse with her husband all the time, her husband would be too accustomed to her, and would eventually be repulsed by her. Therefore, the Torah says that a menstruating woman shall be ritually impure for seven days, during which she is prohibited from engaging in intercourse with her husband, so that when she becomes pure again she will be dear to her husband as at the time when she entered the wedding canopy with him.

Notwithstanding Tosafos and the Kuzari, it seems other Jewish thinkers see the impurity of Niddah as not merely a technical ritualary requirement, but an indication of something spiritually noxious. For example, Ramban (Bereishis 31:35) comments on Rachel’s excuse to Lavan that she could not get up from the saddle (where she was hiding his teraphim) because “I am in the way of women”, i.e. I am menstruating.”  We could simply understand that Rachel was saying she either was feeling too ill to get up, or it might have been hygienically difficult for Lavan to search where she was sitting due to the blood. However, Ramban explains that the ancients (not just Jews) had a spiritual tradition that menstrual blood was harmful and required distancing.

In fact, many cultures throughout the world and time have observed some sort of menstrual taboo, and notably the Ethiopian Jews had menstruating women live in separate quarters, presumably due to this idea (see https://en.wikipedia.org/wiki/Menstruation_hut#:~:text=In%20Ethiopia's%20Jewish%20community%2C%20when,Jewish%20community%20as%20margam%20gojos. ) How do we understand the Ramban and others who have this tradition?

Bereishis Rabbah (17:8) states: 

"Why were women given the mitzvah of niddah?  Because they caused the blood of Adam, the original man, to be spilled."  (If Eve had not convinced Adam to eat from the Tree of Knowledge, then mankind would not have been punished with mortality.) 

The midrash touches upon the theme of blood in the menstrual cycle and sees it as a penance for Eve having brought mortality into the world.  In addition, though this midrash seems to be referring to a punishment or consequence of Eve's sin, it actually does not mention the pains or suffering of the menstrual experience, rather it refers to the mitzvah of observing the laws of niddah.  It would seem the midrash is of the opinion that observing the laws somehow rectifies the introduction of mortality into the world.  The question we might then ask is how exactly do the niddah laws address or counter the force of death in the world?

To begin to find an answer, let us consider that the psychological connection between menstruation and death has already been noted by psychoanalytic theory.  The idea of a woman experiencing a flow of blood from her body automatically suggests a wound of sorts.  The late Karen Horney, a psychoanalyst trained by Sigmund Freud, who wrote extensively about female psychology, suggests that the pain women experience right before and during their periods are more of an emotional  nature than any physical cause.  She attributes this pain to be a somatization of the sadness and mourning felt by each menstruant woman for the loss of a potential child that could have been born (Menstruation and Psychoanalysis by Mary Jane Lupton. 1993. Champaign, IL: University of Illinois Press: 186-187.)  The blood is a potent and symbolic reminder of the potentially conceived child's "death". 

Interestingly, certain American Indian tribes view menstruation itself to be a miniature miscarriage.  Without the benefit of modern scientific knowledge, these Indians quite understandably saw a causal link between the regular periods of a fertile non-pregnant woman and the conspicuous absence of blood during pregnancy.  They therefore surmised that during pregnancy the menstrual blood coagulated and congealed to form a child, which is ultimately born after nine months.  Thus, according to these Indians, a menstrual period is tissue substance from a failed or aborted child.  This anthropological insight further corroborates the idea that menstruation is linked to mortality in the Torah and in other societies, because on an emotional level, what people perceive by observation is as relevant as what is the actual biological fact.

As we have noted in Psychology of the Daf Yevamos 85, often that which is associated with death is considered impure.  Higher standards around exposure to death impurity are observed within the sacrificial order because Cohanim embody the middah of chessed, and in fact, this is how they are more able to activate atonement and forgiveness.  The harsh realities of death and resultant despondency and cynicism about life would disrupt that mode of thought and feeling (Recanti Vayikra 21:1).  The Hebrew word for Tumah or impurity is often used in connection with organic objects that seem to have in some way been spoiled or misused.  For example, if a kosher animal is found dead it is  unkosher for eating as well as tamey. Meaning, that it can render holy foods to be "impure" via contact.  However a kosher animal that is slaughtered according to ritual requirements although it also is dead, is not tamey (Mishna Chulin 4:4.)  This seems to imply that tamey exists as a result of a ritual misuse or less than ideal manifestation of a decaying organic object.  Similarly, we find that all semen that is not absorbed by the vagina and develops into a fetus, also represents a less than ideal manifestation of organic matter (Vayikra 15:16, Gemara Niddah 43a-b, and Rambam, Avos Hatumah 5:1,9.)  Additionally, we find that the diseased Zav and the Leper also are labeled as tamey (Mishna Keylim 1:3-4) since they too are experiencing a corruption or malfunction of their bodies with dreadly or loss of reproductive ability.  This observation, that tumah is connected with potential life and death, is also made by Kuzari (II:60).

Regardless of what the basis of the Niddah is, many women subjectively feel that they are being rejected.  This is understandable, as after all, contact is prohibited and many men, out of fear that they become too aroused, may recoil and excessively distance themselves.  This is a difficult area for some religious couples to negotiate; expert and sensitive halakhic guidance should be sought, especially in a situation of past sexual trauma.  I will not wade into the halakhic details but I will speak of some general ideas. 

In the times of the Temple, any object that a Niddah woman sat or lay upon became an Av Hatumah (a primary source of Tumah, Mishna Zavim 5:6), this must necessitated extreme and difficult measures, especially for families of cohanim that subsisted on Terumah bread and sacred food, which had to be eaten when in a state of ritual purity. Can you imagine dunking your beds, linen, couch, eating utensils and dining room furniture in a mikvah every month?  The fact that there is no record of ancient observant Jews having their menstrual wives live in separate dwellings even with all these complications, unlike many other ancient cultures who observed this taboo, is a testament to a strong and specific will to NOT distance or reject a Niddah woman. 

Interestingly, the Archeologist Yonatan Adler notes the exclusive preponderance of chalk vessels in Jewish settlements dating as far back as the first century CE.  This is an odd and inconvenient technology which was very different from the standard earthenware vessels located in gentile archeological counterparts. It can be explained by the halakha that stone vessels do not become impure.  (“The Origins of Judaism: An Archaeological-Historical Reappraisal”, New Haven, CT 2022, pp. 66-71.)   Thus we see an ancient “Kosher Innovations” product designed specifically to overcome the halakhic challenges of impurity during the Temple Era.

And finally Gemara (Shabbos 64b) records the following dispute: 

וְרַב, מַאי שְׁנָא הָנֵי? אָמַר עוּלָּא: כְּדֵי שֶׁלֹּא תִּתְגַּנֶּה עַל בַּעְלָהּ. כִּדְתַנְיָא: ״וְהַדָּוָה בְּנִדָּתָהּ״ — זְקֵנִים הָרִאשׁוֹנִים אָמְרוּ שֶׁלֹּא תִּכְחוֹל וְלֹא תִּפְקוֹס וְלֹא תִּתְקַשֵּׁט בְּבִגְדֵי צִבְעוֹנִין, עַד שֶׁבָּא רַבִּי עֲקִיבָא וְלִימֵּד: אִם כֵּן — אַתָּה מְגַנָּהּ עַל בַּעְלָהּ, וְנִמְצָא בַּעְלָהּ מְגָרְשָׁהּ. אֶלָּא מַה תַּלְמוּד לוֹמַר: ״וְהַדָּוָה בְּנִדָּתָהּ״ — בְּנִדָּתָהּ תְּהֵא עַד שֶׁתָּבֹא בַּמַּיִם.

The Gemara asks: And according to Rav, what is different about these, the woolen cap and the wig, that the mishna permitted going out into the courtyard with them? Ulla said: So that she will not become unappealing to her husband. That would be the result if all ornamentation was prohibited. As it was taught in a baraisa with regard to the verse: “And of her that is sick in her menstrual status [niddata]” (Leviticus 15:33), the Elders of the early generations said that this verse comes to teach us that the menstruating woman should be distanced from her husband in all senses, like a person ostracized [menudeh] by the Sages. This includes that she may not paint her eyes blue, and she may not rouge [pokeset] her face, and she may not adorn herself with colorful clothing. Until Rabbi Akiva came and taught: If you do so, you are making her unappealing to her husband, and her husband will consequently divorce her. Therefore, extreme strictures should not be instituted. Rather, what is the meaning of that which the verse states: “And of her that is sick in her menstrual status”? She shall remain prohibited in her menstrual status even after the flow of blood has stopped until she immerses in the water of a ritual bath.

In this Gemara, we see a tension between the two trends, either to distance a Niddah due to her impurity or to soften the halakah and prevent her feeling rejected, with the opinion of Rabbi Akiva prevailing.