Had sex with my friend's bitchy wife

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I'm pretty much still baffled days after so I hope this is coherent enough.

I've known my friend since we were kids and I moved here with my family. He went through a pretty rough relationship before settling on his current wife. Someone I never liked from day 1 and I'm sure one of the biggest reasons we stopped being best friends a few years ago (I would always turn down invitations to go over his house because of her).

You know how a person just rubs you the wrong way sometimes. Well take that and multiple it by a thousand and you'll get how I feel about her. I find her ugly, boring, unintelligent and even short conversations drain and exhaust me. She never outright mentioned it but I felt the same from her. I always thought she hated my guts and wanted me out of the picture.
Well, a few days ago, in an unforeseen set of circumstances that involved us having to spend a few hours together, we had sex. And it was the best sex of my life. I mean, it destroyed my concept of good sex. And judging by her reaction I would say she more than enjoyed it as well.

Now comes the obvious question. Do I tell my friend or not? It would probably destroy their marriage (they have a 4yo daughter) and anything left of our friendship but at this point I just want to do what is right for him. And no I don't want to maintain any relationship with this woman whatsoever, what I did was a big mistake.