Human sexual development is caused in part by the sexual hormones testosterone, estrogen, progesterone, oxytocin, and vasopressin. Sexual motivation is partly based on the intellect and the sub-conscious. As such no one has control of their sexual development. The relationships of the factors is not understood. DNA is part of the causes although the nature of the causes is not well understood.
Human sexual development undergoes changes from adolescence to adulthood. So far as homo or hetro sexuality, it is not under conscious control. Many false and inconclusive theories have been presented with no conclusive answers as to the cause of human sexuality although the effects of the hormones is clear enough. An excess or insufficient amount of hormone release causes differences and there are enough differences that sexuality can be represented by a curve that ranges from a little or a lot with most people being in the middle somewhere.
The question of whether or not homosexuality is normal is answerable as yes, homosexuality is normal for homosexuals but not for hetro’s and the same for hetro’s. Sexual behavior does not necessarily follow from the amount of hormones. Neither does sexuality depend entirely on desire which is a result, not a cause.
The things that cause the sex drive are the same for all adults however like eye color, no two people are alike. It’s part of the wonderful experience of being human.