The traditional interpretation, which has tremendous merit, is that one who has crushed or maimed private parts has a pegam, and this pegam, just like that of mamzerut or coming from one of these nations, is sufficient to prevent him from marrying into the kehal Hashem. He can convert if not Jewish, and if already Jewish may marry a giyoret, but not into kehal Hashem.
Another thought, and interpretation occurred to me -- that being wounded in the private parts is not a pegam, but rather a mitigating circumstance. At least in the hava amina. A bastard cannot marry into the kehal Hashem so as not to propagate the mamzerut. And similar, there is this pegam for several generations for the Ammoni and Moavi. But what if someone is evidently, and outwardly infertile. He would not be able to propagate the mamzerut, or the pegam of these other nations. Perhaps then he should be able to marry into kehal Hashem. All he would get out of it would likely be companionship. Could he perhaps marry into kehal Hashem. The answer is a definite no. And the nafka mina is what of a perfectly kosher Israelite who is already in the kehal Hashem? The pasuk would then not be addressing him at all.
These are just my speculations. Obviously not halacha lemaaseh, and I am not really endorsing this as the plausible interpretation of these pesukim.