Only once, right before we split up for a few months, a few years back. Things got really bad for us (turns out he was more or less cheating emotionally, without being aware of it at the time) I began to feel really lonely, he wasn't treating me well at all, and I did start to think about it. At that point is when I realized things were REALLY bad, and I was honest with him. I had no one in particular that I had thought of, just more of a "If this happens to other people, I could understand why they'd cheat" sort of thing. I told him flat out, exactly that. Things got worse from there for us, and we did eventually split up, like I said. I don't think it was a time together issue, it was a communication break down for us, and he had met a "friend" who had lead him a bit astray - rather than focusing on fixing us, he kind of just shut down and turned to her, mostly.
We ironed it all out though, and things have been good since, and I haven't even remotely had that feeling since, and I hope he hasn't, either.