Some very interesting psychological studies have discovered two things about human sexuality that seem to suggest that who you're attracted to is neither a choice, nor does it have anything to do with genetics.
The first one is the discovery that there are measurable differences in different regions of the brain between heterosexual men and heterosexual women. For example, in women, the corpus callosum is much more developed when compared to men. When they added homosexual men and women to this research, they found that homosexual men's brains structurally resembled heterosexual women's brains more closely and vice versa for homosexual women and heterosexual men.
The second one is that brain development during critical periods, such as within your mother's womb, as well as reproductive organ development, is triggered through hormonal changes of the mother. One reason why we have babies being born with seemingly ambiguous genitalia is due to hormonal imbalances. My sister's third son, for example, was born with ambiguous genitalia. Only through DNA testing did they find out he was a boy, so they didn't get all cut happy with the scalpel. But the doctors did notice some hormonal imbalances during the pregnancy that they tried to compensate for by giving her estrogen supplements.
These two pieces of research have created some interesting theories that your sexuality is determined in your mother's womb based on the available quantities and ratios between certain hormones as they are transferred to you from your mother. Some theorists believe that some women develop a sort of immunity (for lack of a better word) to testosterone and floods her system with even more estrogen to compensate while others ability to produce estrogen becomes suppressed. If a baby's bloodstream has higher quantities of estrogen flooding into his brain, it's likely to trigger some developmental and structural growth in some regions while suppressing it in others and causing these measurable differences between men's and women's brains.
If this theory proves to be true, and your sexual preferences are linked to structural differences in your brain, it would indicate that your sexuality is not a choice and not something you can change.