What makes a person bisexual?

Answer 1

Actually, most humans are bisexual, at least in the sense that we're capable of intercourse with both sexes, even if we don't actually do it. It's just that most of us are more strongly attracted to one sex or the other. The vast majority of us are attracted to the opposite sex ("heterosexual"), but a few of us are primarily or exclusively attracted to the same sex ("homosexual"). Another relatively small group of us are attracted more or less equally to men and women ("bisexual").

Sexual orientation appears to be determined by the release of certain chemicals in the womb during fetal development, and fixed by the age of six at the latest. We don't have any more control over our sexual orientation than our hair color.