The qaulity of infidelity is a pretty big clue that it's a bad idea... The fact that she would get emotionally involved with you while she is commited to someone should be a deal breaker... It's a lack of a conscience that she is knowingly causing pain to her partner, and being untruthful about her feelings. No one deserves that. It's unlikely it'll be different with you, but let's play that card. Even if it is different with you, are you okay with someone who could do that to someone they are suppose to love?
If you are unhappy in a relationship that is no excuse to leave it by infidelity and such. In the same way that it's not right to invade into a relationship because the couple is unhappy... It's the cowards way out, if she doesn't want to be with that woman she should have left her, regardless of having you waiting on the side or not.
I don't understand why people just don't do the "right" thing, when i'm sure almost everyone knows what it is.