Okay, i aint no expert on this topic, BUT i have heard in my training that sex/masturbation is not the best of ideas before a fight.
Its apparently the same before all sports. I used to hear it before a match in AFL (Aussie Rules Footy) all the time.

Here is how i would see it as being a bad idea...
(and considering this will be my second post...jeez what a way to enter this board)

Most sexual encounters and (preferably) in cases of masturbation an orgasm will occur (yea not shit
SuX0rZ, get to the point).
When you orgasm, your body releases an enzyme/hormone called Prolactin which is secreted from a section in your brain called the adenohypophysis (anterior pituitary gland).

Now, The major effect of increased prolactin is a decrease in normal levels of sex hormones i.e. estrogen in women and testosterone in men

So thats why, when after you've had sex, your penis goes into what is called a "refractory stage". Basically, its when you get a limp dick. Funny fact eh?

Now, as we all know, testosterone is vital in making us strong/hyper/bloodthirsty etc etc. It helps us go from boys to men.
Testosterone is a steroid hormone from a group of hormones called the androgen group. Androgens promote protein synthesis and growth of those tissues which give us amazing amounts of energy (of which most is expelled via creation of heat (anabolic effect).
Anabolic effects include growth of muscle mass and strength, increased bone density and strength, and stimulation of linear growth and bone maturation.

SO!!! if you've missed the point.

1. An orgasm lowers your levels of testosterone
2. Testosterone is vital in making us strong and energetic
-----------------------------------------------------
Conc: Don't have sex before a fight because your body will not be able to maintain the same strength etc etc.

HOPE THAT HELPED.

I did biology in my final year...