Does sex make you fat?
No...sex is exercise. It burns lots of calories.
If the man has fat, sex will make him LOSE weight. You would be surprised how many calories are burned during sex. If the man has no fat, and little muscle, he might Gain weight. This is because the exercise may build his muscle mass, which has increased weight. Either way, it is good for you unless you catch a disease or get someone pregnant or do something illegal (like statutory rape).
Sex hormones produce secondary sex characteristics. These in turn produce sex cells (sperm and eggs) which will produce a new individual. In males a beard, deep voice, larger muscles, sex drive and sperm production are secondary sex characteristics. In females, these are wider hips, breast development, fat deposits under the skin (very thin women don't make good fetus incubators), sex drive and egg development with uterus development.