Unfortunately I can't provide sources at the moment (Luke probably can), but I have seen research both sociological and anthropological showing that women and female higher primates in general have a tendency to try to mate with multiple dominate highly masculine males, sometimes secretly, while they tend to have long term pairings with less dominate, less masculine males. The theory is that the genes of the more masculine men lead to more fecund offspring, while the parenting of the less masculine men leads to higher offspring survival. In society this works out to women dating more masculine men (and testosterone is of course linked to the aggressiveness and risk taking we associate with "bad boys") prior to marriage, and then marrying less masculine men (nice guys). And if they cheat, they tend to cheat with "bad boys" and have their "nice guys" raise those kids.

EDIT: For pure anecdote, I am a nice guy (I think) who always complained about the "bad boy" thing, and now I am raising a step-daughter from my wife's youthful short term relationship with a guy everyone would still call a "bad boy." My wife is winning at natural selection! As is that jerk :(