Sex is a carnal instinct and a key motivating factor for many men to hit the gym, in order to exhibit a fit body and portray themselves as good performers, to the opposite sex. But many body builders and athletes often fear that sex will impact their performance in the gym and hamper the muscle growth. There is definitely a co-relation between the two; but not necessarily a negative one.
What generally tends to happen is that, sex and muscle growth require and consume high levels of testosterone. But an active sex life, signals the body to increases the production of testosterone. An increase in this hormone leads to better and faster development of muscles. This in turn means that the more sex you have, the better your performance will be in the gym - while lifting weights. The other benefit of higher testosterone levels in the body is the increase in libido and higher sex drive. Hence regular workout improves your sex life, and regular sex enhances your performance in gym.