Actually, I can see Leonard's Mom as a youngish student getting involved with a somewhat older Prof or something like that. That was very common back in the day (And it still goes on, of course, although now it is frowned upon- back in the 60s/70s when things were more freewheeling it wasn't, particularly). I can see that more than I can see her getting involved with a student of a similar age. And it would explain why he was attracted to her.