Men also don't tend to bond irrevocably through sex; not as quickly as women.
Look at this this way -- a couple, Roger and Karen.
Karen discovers Roger has been cheating on her.
"Roger, how *could* you!!?"
"Believe me, babe, it was just a fling, she meant nothing to me!"
But Roger discovers Karen has been cheating.
He doesn't even ask. They both know it's over.
It would have been interesting in human history if women had been able to hold men to the same standards as men have for women.