Sexuality is complex thing. It comes down to biology (that being what we naturally want to do, whats in our instincts) and personality (the part of us shaped by environment, upbringing, social order, per-pressure).
People are shaped on what they have in them to do, and what they are shaped by around them, so its kind of that struggle between internal and external parts of us. Like the young gay muslim wants to kiss the hot man before him, but doing so would have he pelted with rocks by the religious faith dominating the world around him, or the woman married to a man all her life, living a lie, wanting to be herself and see women, but afraid of being alone if she does.
Its also complex in that sexuality isn't just a three card system, rather it can have many different variants and shades, it isn't set on just one, as many people might want to experiment, might want to go between different kinds of people, try all things, try and mixture. There are no real standards to it, despite what most of society has already declared it to be.