The Torah explains the reason for the prohibition against accepting male converts from Amon or Moav is because of their failure to bring food and wine to Bnei Yisrael in their time of need and the fact that they hired Bilam to curse Bnei Yisrael. Women are excluded from the prohibition. The Bavli explains that Amoni and Moavi women obeyed social norms of modesty and could not be expected to leave their home to bring supplies; therefore they are not blamed.
The Yerushalmi is not satisfied with this explanation (Yevamos 8:3). Even if women would not come out to serve men, why, asks the Yerushalmi, did the women not come out to bring food to the women of Bnei Yisrael? The Yerushalmi answers that the prohibition against the men depends also on their having hired Bilam. Since men alone and not women had the power to advise on this plot and initiate it, only they are blamed and punished.
The meforshim on the Yerushalmi ask: if the trigger for the prohibition against the men was the hiring of Bilam, then it should follow that only Moav and not Amon should be blamed, as only Moav was involved in this plot?
Ramban interestingly reads the pasuk as offering two separate reasons: Amon was punished for the failure to offer supplies; Moav was punished for hiring Bilam. However, the Bavli (Sanhedrin 103) indicates that it was over a matter of food that both Amon and Moav were punished. I'm not sure that the Ramban need necessarily account for that aggadita in his reading of pshat, especially given that his reading is very close to that of the Yerushalmi (which he references).
The Ramban, however you square his reading with the gemara, avoids the potential pitfall of applying both reasons in the pasuk equally to both Amon and Moav, which seems like overkill – why is one reason alone not sufficient to justify either nation’s punishment? From a musar perspective one might suggest (my wife’s idea) that the lesson here is that the two reasons are in fact equivalent – the lack of midos in failing to provide supplies is no less a crime than the hiring of Bilam to offer his curses. Other solutions are offered as well. The Dubno Maggid has a clever approach: Amon/Moav might have justified their failure to help Bnei Yisrael by claiming that supplies are valuable and they cannot afford the drain on their GDP to help other nations. Yet, as the pasuk immediately notes, these same nations of Amon and Moav had the resources and wealth to hire Bilam to curse Bnei Yisrael! Their very actions negate any claims for mercy they might advance. The lesson here speaks to us as well. The person who claims he has no time to learn but somehow finds time to spend elsewhere, or who claims no means to donate to charity but finds the means for a new car or a lavish vacation, negates his own arguments.