Because I shit from there and probably don't wipe well enough for my wife to want to risk inserting her fingers or whatever inside.
I can't understand how it feels good either, since I once had to jam a foreign object up my asshole to unplug myself and become regular again.
To each his/her own I suppose.
I'm about to blow your mind: if you jam something up a girl's vagina when she's not in a sexual mood, it doesn't feel good, either.
Unless you think girls are cumming every time they insert tampons or something.