An indelicate question...
He was male, and a wounded veteran of the gender wars. I have no idea if the question ever occurs to women, although perhaps it's the male equivalent of, "Why are women attracted to bad boys?"
I thought about it for a few days, and came up with a possible explanation:
The way you get good at sex, is the same way you get good at anything. You practice.
(Perhaps with the aid of any number of the how-to books available.)
There are two ways you can practice. You can find a willing partner, and practice a lot together.
Or, if you can, practice with a lot of different partners. This is obviously easier for women.
Observation: People capable of forming stable, long-term relationships - do.
Corollary: Most of the people capable of forming stable, long-term relationships are going to be in one at any given time.
So... the people you are most likely to meet who are great in bed, are most likely to come from that subset of the category of "great lays" who are not capable of forming stable, long-term relationships, i.e. "disasters as human beings."
I thought this was one of the cleverest short arguments I'd ever come up with - and most depressing.
My friend couldn't find any holes in it either.
And by the way, he died very lonely.