If she is married, don't do it. Same with fiancee. If it is just a BF, then find some really good reason (Drunk, high, etc.) that you did it. Then send the guy a letter explaining and saying sorry. The last part is a save your own skin precaution. BTW, say his GF was drunk too. That would make her have a reason also.
Asamin, that's not good reason at all.
-----------
Only way I'd hit it is if I didn't know (it would be an 'Oh, shit' situation) or if she broke up with him.