Thank you for your thoughtful response.
My reference point is the Kinsey Scale. Alfred Kinsey studied human sexual behavior in the 1940's and developed this scale in 1948. It is an attempt to categorize the heterosexual-homosexual continuum.
Rating Description
0 Exclusively heterosexual
1 Predominantly heterosexual, only incidentally homosexual
2 Predominantly heterosexual, but more than incidentally homosexual
3 Equally heterosexual and homosexual
4 Predominantly homosexual, but more than incidentally heterosexual
5 Predominantly homosexual, only incidentally heterosexual
6 Exclusively homosexual
He introduced this scale with the comment:
"Males do not represent two discrete populations, heterosexual and homosexual. The world is not to be divided into sheep and goats. It is a fundamental of taxonomy that nature rarely deals with discrete categories... The living world is a continuum in each and every one of its aspects.
While emphasizing the continuity of the gradations between exclusively heterosexual and exclusively homosexual histories, it has seemed desirable to develop some sort of classification which could be based on the relative amounts of heterosexual and homosexual experience or response in each history. An individual may be assigned a position on this scale, for each period in his life. A seven-point scale comes nearer to showing the many gradations that actually exist."
In Kinsey's scale he was primarily counting only actual sexual encounters. Sexual attraction, desire and libido originates in the brain and precedes actual sexual acts. As an example, a young man could be highly and exclusively sexually attracted to woman but has never had the opportunity to make that desire a reality. Even though he has never engaged in sex with a woman, he would still be considered heterosexual.
Recognizing that human sexuality is more complicated than Kinsey's scale, in 2005 the American Psychological Association tried to broaden his approach with,
"Sexual orientation is an enduring emotional, romantic, sexual, or affectionate attraction toward others. It is easily distinguished from other components of sexuality including biological sex, gender identity (the psychological sense of being male or female), and the social gender role (adherence to cultural norms for feminine and masculine behavior).
Sexual orientation exists along a continuum that ranges from exclusive heterosexuality to exclusive homosexuality and includes various forms of bisexuality."
So being exclusively heterosexual or exclusively homosexual brackets a large sliding scale of bisexual variations. And people's sexuality can change over time, moving up and down the scale between both extremes. A good example is my current partner. She was exclusively heterosexual during her youth but in her late 20's had some homosexual encounters. After experimenting with bisexuality for a while she decided that heterosexuality was her preference and has remained so ever since. So her position on the Kinsey scale would have been different during different times of her life. Human sexuality is therefore very fluid, dynamic and complicated and thus leads to an infinite number of variations, but unfortunately humans have little capacity for random and unstructured thought so in a attempt to make sense of this, labels are created to organize people into tidy little boxes.
We all know that that paradigm is doomed to failure. I think labels, and societies insistence on them, is the root of the problem here. (The questions I raised at the end of my original post are indeed rhetorical.) As much as I dislike labels we are unfortunately stuck with them. Maybe instead of using the euphemisms, "gay" and "straight", we should all use our positions on the Kinsey scale.
If a man is married to a woman for 20 years and exclusively has sex with her, he is heterosexual. If he now gets the desire to occasionally have sex with men while still having sex with his wife, he would then be bisexual. If he finds that he prefers sex with men and eventually divorces his wife and has sex with men exclusively, he would now be homosexual. That all is straightforward.
I don't think that anyone, no matter where they fit on the heterosexual-homosexual continuum, should have to defend their position on that scale. The "threatened" feeling that you describe heterosexual men as having I think of more as "frightened". Frightened by societal pressures and expectations. Returning to my example above; If a man is married to a woman for 20 years and has always been heterosexual but now has a desire to have sex with a man just to see what it is like, he is frightened that he will be labeled "homosexual" and ostracized by society even if he has just one encounter. That is very unfortunate and I think the primary reason that heterosexual men often strongly defend their heterosexuality even if they have other desires. For some reason, society tolerates the occasional homosexual encounter in a heterosexual woman, and (as you referred to) the occasional heterosexual encounter in a homosexual man, but is very rigid in it's expectations of sexual behavior in a heterosexual man. And that is because many of societies "rules" were created by heterosexual men.