Tempting offer

Since you're much fitter, better looking and more intelligent than your roommate, it makes more sense for you to be the one who gets his girlfriend pregnant.

That's probably exactly what his girlfriend was thinking when she asked him to make you the tempting offer in order to "secure better genes for their child".

Of course, you accepted the offer. His girlfriend has now also moved in with you and your roommate temporarily. Of course, she sleeps in your bed and your roommate can listen to the sounds of you and her making a baby at night.