I'm surprised no one mentioned Faster Than A Kiss, though the age difference isn't that large. I believe he's in his early 20s and she's a teenager. They're "married" but in typical shoujo fashion, they haven't really consummated their marriage. He's also her teacher too (yah I'm a sucker for teacher/student shoujos).

Toriko Aigan Shoujo, seriously, it's a bit extreme for me at times.

Koi Kaze (incest...)

Incest♥Necrophilia♥Selfcest