I've found I can't even make choices about who I am attracted to. Sometimes I'll have urges for men who logically should be repulsive to me (generally when I'm attracted to a woman though, it makes sense, she's usually hawt!
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Most men and women seem to be genetically hardwired to at least prefer humans of the opposite sex. I'm going to go with no choice here about natural preference. Social pressure may influence someone to deny preferences, which is where the element of choice may come into play.