Our Gemara on Amud Beis quotes the verse in Vayikra 19:29:

אַל־תְּחַלֵּ֥ל אֶֽת־בִּתְּךָ֖ לְהַזְנוֹתָ֑הּ וְלֹא־תִזְנֶ֣ה הָאָ֔רֶץ וּמָלְאָ֥ה הָאָ֖רֶץ זִמָּֽה

Do not degrade your daughter and make her a harlot, lest the land fall into harlotry and the land be filled with depravity.

Rashi explains the verse as follows:

Scripture speaks of one who gives his unmarried daughter away for illegitimate concubinage (cf. Sifra, Kedoshim, Chapter 7 2; Sanhedrin 76a).

Ramban objects to Rashi’s peshat:

Ramban says that halakhically the word “zonah” is not used in reference to sexual relations outside of wedlock. Halakhically, a single woman who had sexual relations with a permitted single man will not have the status of “zonah” (I’ll translate zonah as a person who is sexually promiscuous, and the word that zenus as sexual promiscuity,) Since she is not halakhically a zonah, she is still be permitted to marry a Cohen. One only gets the status of “zonah” if one has relations with a person to whom marriage is impossible (gentile or relatives that incur kares.) Therefore, Ramban interprets this verse as a prohibition against a father, or even the daughter volitionally, promoting and engaging in a forbidden sexual relationship.

Gur Aryeh defends Rashi’s peshat as follows: The vernacular term “zonah” or “znus” is not a halakhic one. Just as clinical terms such as depression or obsession are different than the commonly spoken terms, so too halakhic terms are different than linguistic expressions. Thus, zenus means general sexual immorality, and so the verse can be understood as a prohibition against fathers prostituting their daughters. (In not polite terms, “pimping” them, which sadly does exist in certain cultures and eras.)

The defense of Rashi is so reasonable that it is difficult to understand what the Ramban’s objection was. We might say that the Ramban requires linguistic consistency and could not accept the Torah using the word zonah in a fuzzy, imprecise manner. Indeed, Chazal often bring proofs for definitions of words and their halakhic implications throughout Tanach, despite context. The rules of interpretation of Torah might operate along these lines, that each divine word is used in a consistent manner that has meanings across contexts. 

I also wonder if it has to do with the famous halakhic debate between Ramban and Rambam regarding the permissibility of a pilegesh. Rav Yaakov Emden analyzes the dispute in great detail in a responsum (Yaavetz II:15). A pilegesh is a woman whom there is no marriage contract, nonetheless she is in a monogamous relationship so it is not considered promiscuous nor are the questions of mixed paternity. The essential point is that the Rambam holds a pilegesh is only allowed for a king. It is difficult to know what Rambam’s source is, as there is no clear indication of this halakhic principle in the Gemara. However, based on the Ramban, Rav Emden says so long as this woman is monogamously committed to one man, it is permitted. Of course, she cannot be a Niddah. 

Now no credible posek would ever agree with Rav Emden’s analysis on a practical halakhic basis for obvious reasons.  We know how easily such practices would degrade into promiscuity and chaos. On a practical level, who would follow all the laws of Niddah, and stay monogamous? If a couple could manage that, they might as well be married. Yet, even so, since it is technically permitted, this could explain the Ramban’s reluctance to define this prohibition as having anything to do with sexual relations with an otherwise permitted single woman out of marriage. Interestingly though, Rav Emden still offers a way to learn the verse similar to Rashi. Because if the father prostitutes his daughter out, surely it’s not monogamous and considered an act of zenus and promiscuity.

While we are on the topic of ethical and moral obligations regarding sexuality outside of marriage, Rama (EH 177:5) rules that if one already did have relations with an otherwise permitted single woman outside of marriage, it is a mitzvah to marry her. This applies even if she was a niddah at the time, and the relationship was developed by route of a major transgression (see Pisechei Teshuva, Op. Cit.).  One additional point, if the woman became pregnant and insisted that she was not promiscuous with other partners, she could demand child support.  Pisechei Teshuva (ibid) says that the judges must use their discretion to determine if her claims seem credible, however if they do seem credible, if he denies paternity he must make an oath, or pay up.