...
I guess it's possible that if you were able to get him married to a lustful woman and then have a lustful man in your court, there's a chance that they could procreate... and in some cases, perhaps the child could be mistaken for yours (in reality, it doesn't make sense for a eunuch to make that mistake, but I'm not sure the in-game event checks that -- maybe someone else knows?)...
Actually this is doable as it happened to me in my last session - but she was not only Lustful but also Hedonist so +40% fertility. She had a child not mine fairly quickly and I legitimized it. Ah, but Eunuchs can't marry...