I know exactly what you are thinking from the subject - its a no brainer! We are 26 (me) and 27, when we got together it was a total sex fest - I was getting woken up in the middle of the night! The rate of sex dropped off after maybe 6 months - which is pretty normal I suppose, but it quickly became so infrequent that I began to wonder...
A few months ago I found out that he had been sufing some gay cruising site and talking to guys in the local area, on further investgation I found out that he had taken advantage of a 'snow day' around xmas last year to meet up with a couple of them, but he swears no full sex was involved and there was just some fumbling - which I believe. His explanation for this little 'indiscresion' was that he wants to experiment before he gets to 30, and he was feeling wierd about getting his first proper job and us moving in together; which I am also ok with (just about).
The big problem is this - I now cannot remember the last time we had sex, I really don't know when it was! However his sex drive hasn't gone, he just expects me to make him come every night and then rolls over and go to sleep. I have tried teasing him, talking to him, refusing to play and nothing works. I love him deeply, we were friends for a ling time before we were a couple, I can't bear the thought of ditching him, but can't help feel like I am being used.