I was in a similar situation to yourself
We were together for about 2 1/2 years
About 6 months into the relationship, sex started to decline, I tried to be understanding as she said it was painful (like a donkey) and so I didnt try it on, waited for her to make the first move etc etc. ended up waiting months at a time before I actually got fed up and tried to make a move myself, then she would have a go at me for "pressuring" her. I tried to talk to her about it, but each time she made me feel like I was expecting too much and that "sex isnt everything" (btw just before someone asks, she was 21, and not a virgin!

)
In the end we broke up, turns out shes a liar and Im better off without.
If I was you mate, I would get out of it, or give her an ultimatum.
I know that sounds weird, its just sex right?!? WRONG, its an important part of a healthy relationship, and if its not happening without fair reason, then something is going on.
Dont spend the rest of your years celebate man, if you wanted to do that you would join a monestry!