I had this issue with my fiance in the first six months we were together. His profile was still up, and I could see that he'd logged on in the time we'd been together (although I think it was only twice). I was in the process of deleting mine.
I can understand how you feel, though. I was cheated on in my last relationship, so when I saw that, I had a huge knot in my stomach. Turns out he had a good reason.