A. Drank too much and cant get it up.
B. Is terrified of having sex perhaps for medical reasons (ie, fear of premature ejaculation or has had some type of accident or surgery)
C. Has some type of STD like herpes that he is scared he would pass on to you.
Sometimes it gets really awkward when you encounter one of these unfuckers on a one night stand, waiting and waiting for the punchline... but it never comes.
Tina: What? He unfucked you too?? Gosh, that guy totally unfucked me, and I was sure I was getting laid. I think he is unfucking that little Turkish girl now. Poor thing, soon she will find out hes an unfucker too.