I've tried to put an honest answer to this...
I've come up with:

*- it's tighter
*- it's taboo
EDIT:
*- bragging rights

but I think the honest truth is that men will put their cocks almost anywhere
(likeajackhammer, Wed 27 May 2009, 16:01, archived)

There's also the
*nudge*
"you see her ? I've had her up the shitter" factor
( Goatworrier had your mum and she was shit, Wed 27 May 2009, 16:03, archived)

Always a proud moment for a bloke.
Especially if she is out with her hubby.
( Cleisthenes morning sickness is a bitch, Wed 27 May 2009, 16:09, archived)