This is one of the laws of social physics. When a man observes a couple and wants to have sex with the female, all the man has to do is simply fight the other male. If he wins, the female immediately forgets about her lover and is forced by nature to have sexual intercourse with this new man. The man will now receive his 'payment' for pulling off such a random and brave stunt. This was discovered when an unknown man witnessed Chuck Norris execute this act and tried to reenact it. Much to his surprise, it worked. This was one of the first laws of social physics ever to be discovered. This also works for women, however, the fight must take place in a kiddie pool filled with either mud or some other slippery substance.
Jim: Hey. I just beat the shit out of your boyfriend.
Sarah: Who? Nevermind. I don't care. Lets have sex!