That's a truly interested and weird question all at the same time. I don't know if you're aware of the website called fml.com (f*%k my life.com). And I do find it quiet interesting to see how many guys have actually slept with their girlfriend's mother.
As a guy I can't quiet understand it myself, but I can understand that it gives some guys bragging rights with their buddies, most probably like when a girl that sleeps with a guy who is quiet a few years older or even younger. Doing this might provoke excitement something that's so weird but in some way attractive to him.
You do also have to remember that it takes two to tango, so it wouldn't only be the guys fault in this case. The girlfriends mother i'm sure would have a played a certain role in this too.
I hope it helps you out a little. To be honest it's a difficult question to answer.