Our Gemara on Amud Beis tells us Er’s motivation to prevent Tamar’s pregnancy was so that her beauty not be sullied from the rigors of pregnancy and childbirth. From the reading of this Gemara one might draw the conclusion that the idea itself of concern about the rigors of pregnancy and the impact on beauty is an unworthy thought. As if to say, we should be above all those base physical concerns. However, Bereishis Rabbah (45:4) indicates otherwise.

וְלָמָּה נִתְעַקְּרוּ הָאִמָּהוֹת, רַבִּי לֵוִי מִשֵּׁם רַבִּי שִׁילָא דִּכְפַר תְּמַרְתָּא וְרַבִּי חֶלְבּוֹ בְּשֵׁם רַבִּי יוֹחָנָן שֶׁהַקָּדוֹשׁ בָּרוּךְ הוּא מִתְאַוֶּה לִתְפִלָּתָן וּמִתְאַוֶּה לְשִׂיחָתָן, שֶׁנֶּאֱמַר (שיר השירים ב, יד): יוֹנָתִי בְּחַגְוֵי הַסֶּלַע, יוֹנָתִי בְּחַגְוֵי, לָמָּה עִקַּרְתִּי אֶתְכֶם, בִּשְׁבִיל (שיר השירים ב, יד): הַרְאִינִי אֶת מַרְאַיִךְ הַשְׁמִיעִנִי אֶת קוֹלֵךְ. רַבִּי עֲזַרְיָה מִשּׁוּם רַבִּי יוֹחָנָן בַּר פַּפָּא כְּדֵי שֶׁיִּהְיוּ מִתְרַפְּקוֹת עַל בַּעֲלֵיהֶן בְּנוֹיָן. רַבִּי הוּנָא מִשֵּׁם רַבִּי חִיָּא בַּר אַבָּא כְּדֵי שֶׁיֵּצְאוּ רֹב הַשָּׁנִים בְּלֹא שִׁעְבּוּד. רַבִּי הוּנָא וְרַבִּי אָבוּן בְּשֵׁם רַבִּי מֵאִיר אָמַר כְּדֵי שֶׁיֵּהָנוּ בַּעֲלֵיהֶן מֵהֶן, שֶׁכָּל זְמַן שֶׁהָאִשָּׁה מְקַבֶּלֶת עֻבָּרִין הִיא מִתְכַּעֶרֶת וּמִתְעַזֶּבֶת, שֶׁכָּל תִּשְׁעִים שָׁנָה שֶׁלֹא יָלְדָה שָׂרָה הָיְתָה כְּכַלָּה בְּתוֹךְ חֻפָּתָהּ, 

Why were the matriarchs barren? Rabbi Levi said in Rabbi Shila’s name and Rabbi Chelbo in R. Yochanan’s name: Because the Holy One of Blessing yearns for their prayers and supplications, as it is written 'O my dove, you on the clefts of the rock let Me see your face, let Me hear your voice' (Song of Songs 2:14): Why did I make you barren? In order to 'see your face... hear your voice'. 

Rabbi Azariah said in the name of Rabbi Yochanan bar Papa: So that they would be clinging to their husbands on account of their beauty. 

Rabbi Huna in the name of Rabbi Hiya bar Abba said: So that they might pass the greater part of their life without hard work. Rabbi Huna and R. Avun in the name of Rabbi Meir: So that their husbands might derive pleasure from them, for when a woman is with child she is disfigured and does not care for her appearance. The ninety years that Sarah did not bear she was like a bride in her canopy. 

The Midrash offers three opinions as to why the matriarchs were barren. The first opinion is the conventional one, that we have heard throughout grade school: God sometimes creates circumstances that cause humans to engage Him in prayer, for the sake of helping humans recognize their frailty and need. The last two opinions seem to be saying the same thing, but there is a subtle difference. Rabbi Azariah and Rabbi Yochanan are focusing on the experience of the matriarchs, that is, they will FEEL beautiful and therefore become more attached and amorous (see commentary of Yfei Toar and Ydei Moshe). This opinion is not comfortable with expressing the Forefathers’ personal desires, and portrays them as ascetic. However, the third opinion is unambiguous in stating that God wanted to preserve the Matriarchs’ appearance for the benefit and pleasure of the Patriarchs.

We see from the above Midrash, that at least according to some opinions, there is great value in maintaining beauty and appearance despite the rigors of childbirth. Er’s sin was not his thinking or his wishes, but his taking action into his own hands to thwart the way of the world and avoid having children for selfish reasons.