It's not always true that they will cheat again, however, if she cheated on this guy just before she met you and then you got with her and you get worried now--why did you get in the relationship to begin with?

I can understand the jealousy part, but it seems like you both have issues going on on both sides. Dancing is just a symbol of the cheating, jealousy--not the actual thing. It could be baseball card collecting that would set you off.

Can you talk to her, a honest, goodness one on one conversation about your hopes and fears?