For the longest time, coaches have advised their athletes to avoid lovemaking the night before a competition, under 2 assumptions:

1. Sex saps the physical (and perhaps even mental) energy that needs to be conserved for the sporting event.
2. Abstaining from sexual activity raises testosterone levels, which, in turn, increases aggression—a key ingredient for participation in professional sports. (In other words, being horny helps you be a better athlete).

It's no surprise, therefore, that some famous athletes abstain from sex for weeks before a big event. However, the only available research on the topic suggests that all this is a myth.

1. A 1995 study† looked at athletic performance on two different days: one day after the men had sex the night before and one day after they had avoided sex the night before. Athletic performance was the same on both occasions. (The study, however, had only 11 subjects, which is a problem because a small sample size can't adequately account for individual psychological differences [eg, temperament, personality traits] which may mediate the effects of sex on athletic performance.)
   † Boone, J.S., & Gilmore, S. (1995). Effects of sexual intercourse on maximal aerobic power, oxygen pulse, and double product in male sedentary subjects. Journal of Sports Medicine and Physical Fitness, 35 (3), 214-217.

2. A 1999 study† found that testosterone levels increased as sexual activity increased. A reasonable conclusion here might be that sex the night before a sporting event might actually improve athletic performance the next day by increasing testosterone-mediated aggression.
   † Salvadora, A., Suay, F., Martinez-Sanchis, S., Simon, V.M., & Brain, P.F. (1999). Correlating testosterone and fighting in male participants in judo contests. Physiological Behavior, 68 (1-2), 205-209.

Some of my own observations:

1. Abstaining from sexual activity might, for some athletes, impede performance by acting as a distraction. If an athlete is excessively horny, he may be more focused on getting laid as opposed to winning his game.

2. Abstaining from sex might, for other athletes, improve performance via sublimation. If pent-up sexual urges can't be expressed directly, they will find a way out in another, more situationally appropriate or useful manner: through work, art, music, or, yes, sports.

3. Sexual activity physically relaxes, rejuvenates, and replenishes the body. It may be almost as vital as sleep, and, in this sense, it may help prepare the athlete's body for a good athletic performance.

Over all, it seems sex before a big game has, at worst, no big effect on athletic performance and, at best, may actually provide some benefit. Athletes should be their own scientists and experiment to see whether their performance differs after sexual activity and after abstinence from sexual activity. Different people will probably find different results.