Show, it’s printed in the beginning of Sefer Chasidim. Most things are without a reason and some seem to contradict open Gemaros. He made a Sefer Hakavod that explains it but we don’t have it.
Sam, are you perhaps confusing this with the Gemara about marrying a niece? Funny thing about that, though, is that you’d probably advise people against that.
I don’t find it to be a Kasha, that it used to be Muttar. Many things used to be Muttar.