Their are a lot of arguements and rumours about not having a sex before a fight or sports match or things like that because it can affect your performance, but then i've heard a LOT of great sports people say that they love to have sex before a match...
I think a lot more things affect your performance that what having sex could.
Surely having sex 10 minutes before you'll be pretty worn out *if your doing it right*, but the night before, it'd be pretty hard to honestly say that you have not fully recovered from the physical side of having sex. Unless it's like crazy 8 hour balancing on 1 arm and your head for 10 minutes wild position sex.... But if you're at such a competitive level of any sport where you actually NEED to worry that much about things that will affect your performance, then you should be fit enough to be able to handle a little sex and recover from it by the next day...
And taking into account other things that can have a LOT bigger affect on your performance, it puts sex or not the night before a fight kind of irrelevant.
Like whether you are loose or uptight before a fight. Nervous or relaxed. Angry or calm. Tired or alert. You know, having a disagreement with your girlfriend 20 minutes before a fight would have WAY more impact (in my opinion) on how you perform than sex the night before.

My 2 cents? I think their are a million more things that could affect your performance more than sex the night before a fight. And even if EVERYTHING else was perfect, I still think it wouldn't make much difference whether you had a bonk or not the previous evening...

With everything, it's just common sense. If having sex means you stay up till 4 in the morning and have a crap sleep, of course it'll affect your performance... But yeah anyways....
That's me done.