here's my theory:
you burn calories while having sex, so if your looking to build muscle, you have to take into account that your burning calories during sex... therefore you must take inadequate calories to combat this problem
my solution:
after you have sex (assuming you can last enough to burn some calories) have a PSM - Post Sex Meal, just as you would have a Post Workout Shake
and of course don't have sex too close to the time of your workout... i used to love to have sex the ***ht before a big game, always got me in the zone