Behavior is always a choice. So the answer is yes, every person chooses whether he/she will have sex and with whom.
Do you remember the time you made the choice to be attracted to men/women? Sex (and therfore sexual attaction) is one of the most basic and primal thoughts of every human. It’s right up there with eating and drinking. We are programmed this way in the same way that we are programmed to eat when we are hungry. Of course, there are people who choose to be gay, but this doesn’t mean everyone does.