IMHO It's not your fault she is married, you are not, she has obviously made the decision to see other people and its her own choice. If maybe you brought up the situation with her maybe you'd find out she is doing what she wants, or has an open relationship, or her husband is a jerk.or maybe they aren't close anyways, a million scenarios.
My point is if its not you it will be someone else. Get in while you can. LMAO.