Descendants of Amon and Moav are disqualified from marrying into the Jewish people. Two reasons are given by the Torah for this prohibition: they didn't bring bread and water to the Jewish people as they traveled near their land, and in addition they hired Bilam to curse them. We can understand why the attempted curse of Bilam was a reason to distance ourselves permanently from those nations responsible for hiring him. If Bilam would have succeeded, it would have meant the annihilation of Bnai Yisroel. It is difficult to comprehend, however, why the lack of hospitality shown by Amon and Moav should be sufficient grounds to prevent us from ever marrying them.
The significance of the inaction of Amon and Moav can be understood on two levels. When Eliezer is searching for a wife for Yitzchok he is looking only for one character trait, i.e. kindness. It is the kindness which Rivkah exhibits by giving water to a traveler that convinces Eliezer that Rivkah is destined to marry Yitzchok. Chesed is so essential to the success of a marriage that the absence of it would disqualify a potential wife for Yitzchok regardless of any other virtue she may posses.
It was a total lack of consideration for others that eventually led to the exclusion of the descendants of the Givonim who joined Bnai Yisroel in the days of Yehoshua. Chazal (Yevamos 78b – 79a) elaborate on the actions of the Givonim. They took brutal revenge on the sons of Shaul because Shaul had indirectly caused them to lose their source of bread and water. After witnessing the brutality of the Givonim, Chazal decreed that since only one who exhibits chesed is worthy to marry into Bnai Yisroel, one may not marry a Givoni. The Givonim who had suffered by having their own source of bread and water cut off should have become more sensitive to others. Because they became completely insensitive, they were no longer considered eligible marriage partners.
Amov and Moav similarly were rejected for marriage because of a complete lack of middas hachesed. As descendants of Lot they should have followed his example of hachnosas orchim and ran to provide bread and water for Bani Yisroel. Chesed is so essential to a Jewish marriage that one who lacks it cannot possibly be eligible to marry into kahal Hashem.
Although the Torah only gives two explicit reasons for the prohibition to marry descendants of Amon and Moav, Rashi (Devarim 23:5) sees a third reason alluded to by the phrase "al dvar" - it was the attempt to seduce Bnai Yisroel into acts of immorality, as occurred with the daughters of Moav, that contributed to exclusion of Amon and Moav. This reason should not be understood as distinct from the reason mentioned in the Torah explicitly. We read in Bamidbar 25:2 that the daughters of Moav seduced the Jewish men to participate in their feasts dedicated to avodah zarah and through this caused them to violate the prohibitions of avodah zarah and zenus. The Moavim couldn't bring bread and water to hungry travelers, yet they were able to invite others to their parties of idolatry and immorality.
It is not a coincidence that those who lacked kindness were steeped in immorality. In Vayikra 20:17 an act of immorality is referred to as chesed. Chesed can mean either kindness or a prohibited marriage because they both stem from the same desire to love and be close to others. Gemilus chasadim is the appropriate expression of this desire, whereas gilui arayos is the distorted expression. One who does not perform chesed may end up showing love in the most inappropriate manner. Amon and Moav who rejected chesed as a way of life, became synonymous with immorality.
The inaction of Amon and Moav in providing Bani Yisroel with food and water indicated a fundamental flaw on two levels with these nations. The lack of consideration for others in and of itself was problematic; combined with the immorality that resulted from it, it rendered Amon and Moav unacceptable to marry into Bnai Yisroel.