There's a whole other thread I did a while ago about ending my relationship with my ex.
I was thinking about something he used to do (well, actually, not do) when we had sex and wondered if anyone else had encountered the same and what might be behind it.
In a nutshell, when we first got together, we had a lot of sex (as you do). It waned a little and then pretty much became a once a month thing towards the end. However, in the year or so we were together, he only came once. Every other time he'd hold back. Every time. 
I didn't really get a straight answer out of him when I asked why. Has anyone else experienced this? No one else I've been with has done this.