Before she cheats...what do you think?

I'm working on a story right now in which a married woman falls for another man, but I'm concerned that readers will dislike her. So my question is this: what would make you feel sympathy for a main character who loves her husband but discovers she has feelings for someone else? Would it depend on how far she took her feelings for the other man? How would you feel if she had an affair with him (slept with him)? What if she only kissed him? Would you feel differently if her husband were abusive? What if he wasn't abusive but instead loved her more than she could ever understand?

I want readers to like this character, but I realize I'm walking a fine line when it comes to adultery (if that's the path she takes). So what do you think: is a cheating main character ever acceptable? I'd appreciate hearing your thoughts on this.