it's an old wives tale although pro bodybuilders still withdrawal from having sex a week prior to a competition.

Sex helps for ANYTHING. I'm not being a smart ass; ask any medical doctor and they will agree. It is good exercise, both physically and for the mind. Not only do you maintain fitness but the endorphins released during orgasm are great.

Depending on how you have sex, you should usually count it towards your "aerobics" aspect of bodybuilding. Since bodybuilding requires more than lifting weights (you need a good diet and aerobics), I wouldn't trade having sex for benching the barbell. I would swap it for 15 min on a treadmill (assuming you take 30 min for sex).

Sex does tend to make you more relaxed and therefore isn't really good for strength exercises.

Sex can actually help when you weightlift. Reason being, that when you have sex, you increase your levels of testosterone. Testosterone is the most quintessential part of your weightlifting experience. It is more powerful than creatine, protein, and HGH. Granted, it is only a small amount of testosterone, smaller than the amount that you would put in with steroids or HGH, it will most definitely not hinder you at all when weightlifting. It also is a cardio workout and is good mentally as well because of the end orphans as the user above pointed out.
Hope this helps!
Monsterbodybuilder