I learned tonight, in reading Sexy Origins and Intimate Things, that the definitions we are used to for sexual orientation (a concept that itself wasn't around until recently) are recent inventions.
Up until the late 1920s, a heterosexual was a person who engaged in "perverted," i.e. non-procreative, sex. This could include same-sex dalliances, as many heterosexuals, by virtue (ha!) of being perverse, supposedly lumped that in. Homosexuals were also in existence, though they were consider a smaller concern. The heteros were the ones to watch out for.
Until within the past eighty years, sex/society was divided not by whom you chose to interact with, but how you did so. Oral sex and birth control were the evil side, regardless of gender.
Fascinating stuff, really.