Vtyjkyuk

The pasuk (Dev 23:4) says that a Moavi and Amoni cannot marry into the Jewish people. The stated reasons are that they did not provide bread and water (as mentioned in Dev 23) and "he" hired Bil'am.

The gemara in Yevamot 76b explains that the injunction is only against the males because they were the ones who should have brought bread and water; it would not have been expected of the women because kol k'vuda bat melech penima, that it isn't proper for women to go out. I'm not sure why the gemara sees this pasuk in tehillim (45:14) as describing the modest behavior of women in the non-Jewish world -- I guess I always assumed that the "bat melech" made it refer to Jewish women, but if the gemara wants to apply it to the actions of the Moavi women, then ok.

But this defense of the women rings hollow because in Bamidbar 25 it was the Moavi women who seduced the Jewish men and drew them to idolatry. Not only were the women guilty of an active evil (as opposed to the men who simply didn't go and make food available) but their action was one which would invalidate any expectation of tzniyut via k'vudah bat melech penima.

So why does the gemara seem to accept this explanation from the Chachamim of Eretz Yisra'el without objection?

The Maharshal in Yam Shel shlomo http://hebrewbooks.org/pdfpager.aspx?req=40936&st=&pgnum=98&hilite= asks this very question that the Moabite woman do not care to be modest and had not encountered the Passuk in Tehillim saying a woman should stay at home. Therefore he says that the Bnos Yisroel were the ones who were modest and didn't go out to ask for bread so the Moabite women were not culpable for not bringing the Jewish woman food, as the Jewish women did not go out from their tents to meet them.

The Midyanite woman or men were never exluded from converting even though they did Zenus, so too doing Zenut alone was not a reason to reject the Moabite woman from converting as they could change their ways once they converted like Rachav Hazonah. Rather as explicitely stated in the passuk they did not do chessed to bring us bread and water when we were on their borders.
This showed a denial of Hakarat hatov because Avraham our forefather saved Lot their forfather from the 4 kings and His zechut saved Lot from the burning of Sedom. So they should of shown a sense of gratitude as their lives were indebted to us as stated in the Ramban on that passuk:

שהיו גמולי חסד מאברהם שהציל אביהם ואמם מן החרב והשבי (בראשית יד טז) ובזכותו שלחם השם מתוך ההפכה (שם יט כט) והיו חייבין לעשות טובה עם ישראל והם עשו עמהם רעה

I asked R' M.Lurai Z"L this question specifically and the answer was that:

The two Parashot (Bnot Moav) and permission to accept Moavites women are based on the same ground - the roots of the Moshiach (King David and his descendants) were concealed in the Moavites women - the story of Rut.

This is the reason Bney Israel approached Bnot Moav in the first place, and even though the price was very steep and thousands died, the Moavites women were Halachicly allowed to convert to allow for Rut.

To your question, the Gmarrah's answer is just a Tirutz (diversion) and it is very problematic as you sensed.