I guess it goes on personal experience. If you feel great and energetic after a good 'workout' with the girl, then maybe before a fight have a good one.
If you are like me, where you feel happy and relaxed, don't feel like moving at all, just enjoy the moments. The I wouldn't suggest having sex before a fight.

WHAT YOU CAN DO THOUGH!

Is get yourself a fine hot girl (make sure she will have sex with you if you win), and make her sit just outside the ring when you're going to fight. That way, you have something to look forward to if you win. It will drive you on in the fight.

Then it's all down to getting home, having a shower and going to the bedroom for another 'bout'.

HAHA