I wrote about this in a previous post, but on the basis of my further reading, thought and self-examination, I am going to stick my neck out (as it were).
It seems to me that all human beings naturally enjoy a degree of same-sex play. All categorical rejections of it are a reflection of internalized homophobia. Same sex erotic response characterizes all plural sexual situations, masturbation and, in men, the widespread interest in pornography featuring transsexuals. These days, when I encounter males troubling to identify as “100% hetero” it raises, I suspect rightly, a red flag.
This being so, it becomes useless to apply either the term bisexual or the term heterosexual to persons whose primary attraction is to the other sex. It similarly becomes useless to apply the term homosexual in the contrary case. This is because neither what we term homosexuality nor what we term heterosexuality is actually about sexual behaviour or narrow erotic/genital response. These terms in fact mistakenly take sexual behavior for the whole of something of which it is simply a part, that is, human bonding behavior, and with which it is also not exclusively associated.
I would question whether the term “heterosexual”, as employed in common parlance, usefully refers to anything at all, beyond signalling latent (or not so latent) homophobia. Bonding behavior between the sexes is the norm in our species. When something is the norm, does it need a name other than in specialized contexts? We recognize the existence of albinos without a corresponding term for “non-albinos” (unless that was it). Similarly, while there are certainly antonyms to blind, deaf, handicapped etc., these are not usually emphasized ad nauseam in a person’s self-description, unless the context requires it. Neither should gays or anyone else have to accept that their bonding behavior is conflated with their sexual behavior and the two are codetermined. A perfectly reasonable alternative with improved etymological purity is, moreover, available: the statistically dominant tendency could be termed “heterotropic” and the other tendencies “homotropic” and “bitropic”.
In any case I think we owe it to true bisexuals to stop using the same word to apply to their bonding behavior and to the normal sexual behavior of heterotropic adults. We also do not need this term for heterotropic adults, because all it vectors for them in most cases is shame and meaningless identity crisis. If you are heterotropic but you sometimes enjoy elements of same-sex play, you are not bisexual, you are just more at ease with your basic nature than most of your peers.