I think I read once that Mohummed Ali didn't have sex for one month leading up to a big fight as it was meant to make him more of an animal and a lot more hungrier and savage in the ring. From this you could deduce that having sex before a fight makes you weak and not so fierce.
the higher you're testosterone levels are the more "crazy" and animal like you get. so therefore they don't have sex to be able to kick serious ass in the ring.