I would have wanted to laugh, too, I'm afraid.
It sounds like she's extremely insecure, and I would venture to guess that she thinks you are cool, and that for whatever reason, percieves that you look down on her. In that case, it seems she took it as a criticism of her for her choice in boyfriends.
I'm curious, have you spoke to her privately for a few minutes? "I realize that sometimes my humor is odd, please, when it hurts your feelings, can you let me explain before you get upset? Often times, it comes off differently than how it really is, and I know your feelings have been hurt by that. I would like to be able to explain to tell you what I really mean, because it is never my intent to mock or hurt you. Would you give me that chance when you think I am making jokes about you, before you accept that assumption?"
If you speak to her in this manner, then you have covered yourself, and if she chooses to continue to be offended, that's her own choice. Once you make it very clear to her that she has taken things out of context without giving you chance to explain yourself, and have made it clear you sincerely don't mean to hurt or offend her, then the ball is in her court.