Synthesesia is a confusion of the senses, not of the emotions or sexual attraction. You could accuse anyone who is confused of having synthesesia if you applied your logic to it, but that doesn't make it any more correct.
I see your point, that it may be similar to synthesesia, in that the brain is confused about what it likes, but I'd like to point out that I certainly am not confused. My brain was hardwired to be attracted to both sexes right from the start, and it has never changed since. I've tried lying to myself about being heterosexual, I've asked myself if I'm gay (being horrified at the possibility because of the rejection that would cause, and being confused because I also like girls), and I never really knew about bisexuality until quite a few years later. Once I found out it struck a chord immediately, and with a little convincing from a friend I admitted to people and then later came out completely.
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