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?