I was confused by that too! It was actually a third couple with the other huge guy though. Still, you'd think that the b-day girl, rather than continuing to hump godzilla until she cant walk, might have wanted to give a shot to that fourth couple that was interested (the couple with the husband that was too small). To each their own I guess.
It sounds like some of the complications in this story may have been avoided if only there were smaller penises involved!