My father cheated on my mother (and on his second wife, and on his mistress with half the ladies in town, until he met his third wife, then died of a heart attack at 51...), and even though I've been married to a wonderful man for twelve years, I still have trust issues. (Thankfully, my husband understands.)
Another thing to consider: only a nutball would be coming on to a man who's married with kids. I'm not saying you're not attractive, but there are plenty of attractive single men. If she knows you're married and she's wanting to schtup you, then she's got some problems, and if you do her, it's gonna get ugly.
If you still have doubts, go rent "Fatal Attraction."