Sex is generally done for pleasure, or to show affection for another, or both. I'd say reproduction as well, but the amount of heterosexual couples in this crossroads seems to be quite minimal.

Many things that are only for pleasure could be considered quite useless. It all depends on the value we lend to them.

In this situation, I would assume one of two things. That you are either asexual and simply frustrated with others constantly having sex, or you yourself are attracted to someone, and feeling a similar kind of frustration.