This is an idea that many people have from pro athletes to gym rats. the idea that having sex the night before may hinder your performance by lowering testosterone and expending energy is well established.
This idea seems to have started with boxers who were told to abstain from having sex even for weeks before a fight in the hope that this might give them the edge in the ring or out of fear that, that edge might be taken away.
The couple of studies that I have been able to find on this subject seem to test things like grip strength VO2 max and aerobic power. But this idea seems to be most prevalent in sports that place a high importance on aggression i.e. boxing or American football. So these tests may not be the best to gauge the effects that can help athletes like aggression.
evidence that states it might actually increse testosterone but nothing conclusive.
So maybe the coaches are not specifically worried about the physiological effects of sex but the psychological effects. The idea behind the psychological effects is that it might lower an athletes aggression and competitive spirt, but on the other hand having sex hte night before might help to reduce strees and relax the athlete which could lead to a better night sleep.
Some coaches state that the reason they make their athletes abstain from sex the night before has nothing to do with the effect sex has on the athlets but it is that having sex the night before or going out looking for sex the night before may cause the athletes to lose sleep and this is shown to adversley effect performance.
Last of all the idea that sex might sap energy from an athlete also seems at least in the sciencetific community is not true as several authors state that sex burns between 30 - 50 calories one even stateing that sex frequently can't be too exausting as many sessions last only 5 minuets. So i guess the moral of this is if your having boring, short sex then this won't affect your performance by reducing the amount of energy you have availble but if your having fun sex for a long time this might.