I am a gay man and as a result I encounter a lot of men who claim to be bisexual. So far, every one of them falls into one of these categories:
Closeted. Gay men who are still in the closet / still have some internalized homophobia. This is particularly prevalent among young men in their early 20s who dated a girl just a year or two ago because that's what everyone in high school was doing. I see this in men of every age, particularly those who come from regions of the world that are particularly homophobic1. These men identify as bisexual because going cold turkey and admitting to themselves they fully aren't who they thought they were or who they wanted to be is too much.
Post Closet Reminiscing. These are very thoroughly gay men who are trying to explain why they tried dating women in high school / college. I know a handful of men who are now married to men and even in the catastrophic event of divorce would continue only to date men. Despite this, they tell me they're bisexual because "I dated a girl once [in high school]." Everyone does dumb things in an attempt to figure out who they are. That's fine, but mistakes along the way don't constitute your long term identity. Periods of change or confusion might be who you were in the past, but they are not who you are now, and that's ok.
Gruff Gays. These are gay men advertising that they are not effeminate. A great example of this is the plethora of men on Grindr who advertise themselves as bisexual. I've met many of them, and none of them are having sex with women. They're simply advertising that they're masculine. I'm not particularly effeminate myself, but I'm sure as hell not bisexual. These two vectors are completely orthogonal.
Burner Fairies. In San Francisco I know a number of very progressive straight men who are trying too hard to be LQBTQ allies. Maybe there was that one time a little something happened at Burning Man when he was on too many drugs. These men are so progressive that it would be an insult to their progressiveness to identify as something so mundane as "straight." Nevertheless, they have only ever dated women and they will only ever date women. Accidents and political soap boxes are not a sexuality.
Attention Seekers. These are straight men who are trying to get attention. A great example of this is Bad Bunny. Doing a line of coke and kissing a man on stage doesn't make one bisexual. Indeed, I've done a line of coke and made out with a woman, but I'm no more sexually fluid than Bad Bunny. Bad Bunny only has girlfriends and I only have boyfriends. Bad Bunny is sexually fluid only to the degree that he is an artist who thrives on controversy. Of course I'm very happy to live in a society where Bad Bunny can identify as whatever he wants. I hope he kisses as many men as he wants. It would be a very boring world where this can't happen, but these things go into the category of "stunts," not in the category of "substantial relationship interests."
Gay for Pay. Obviously there's the category of men who are having sex with men solely for the money. Although overrepresented in online porn, this category of men is remarkably small compared to the above categories. And, unlike the above categories, most of these men don't identify as bisexual, at least not internally. Dating, marrying, or being emotionally intimate with men has never been and will never be on the table for these men. Like Category 5, these men will list themselves as bisexual only to achieve some monetary ends.
My litmus test for bisexuality: Does this man, beyond the age of 25, have a history of dating, and I mean really serious 1+ year relationships, both men and women? I've met tons of men who have dated many women and I've met tons of men who have dated many men, but I've never met a man who has dated women interleaved at the same time he is also dating men. If I were ever to encounter such a man, then maybe I could say I met the first real bisexual man. Ok, the title of this essay is clickbait. I'm sure such a man exists somewhere, but when something is so rare that I've never encountered it, I can say it doesn't exist for me.
I do actually think sexual attraction is a continuous spectrum. I'm particularly fond of the arousal studies where they aim an infrared camera at the genitals and show erotic images of men and women. If blood flow increases to the penis while looking at hot men or hot women, then we can draw the obvious conclusions. But I would claim any man near the 50/50 mark of the spectrum is simply going to date and marry women. If a man is equally attracted to men and women, why on earth would he choose to date a man when it is so much easier to date a woman? There's no need to come out of the closet, no need to deal with parents, no need to throw out religion, no need to fight internal demons, etc. News flash: We all love taking the easy path, and it is so monumentally easier to just date women. It is going to take an extremely lopsided attraction toward men to act on it to the degree that develops relationships.
I'm writing this from the perspective of a single gay man who wants to marry a man. I've found the distinction of "bisexual" to be purely imaginary in this effort. For my own ends I must conclude it does not exist. When a man tells me he's bisexual it means I should promptly move on to someone who is not in one of the above categories so that I might have a chance at a relationship. Identifying as bisexual has become very popular, and like most fads, probably doesn't have much substance to it. Unfortunately, you don't get to choose what arouses you. You can't vibe your way into being bisexual anymore than I can vibe my way into being heterosexual. The men who are lucky enough to be born aroused by both men and women are simply dating women.
An Anthropologist on Mars
We look at and think about things that we don't act on. I think that distinction is very important, but the Categories 1-6 Bisexuals are implying things that they aren't actually going to act on and for that reason I don't find the label "bisexual" to be useful. Here is a great example of things you look at and think about but don't act on:
Eye tracking study from the Nielsen/Norman Group. Men are checking out other men's crotches, but that doesn't make them bisexual. Men also tend to look more at private parts of animals when shown American Kennel Club photos, but that doesn't imply they're likely to marry animals.
Not one single woman looked at his dick. How very dare they.
Footnotes
Particularly homophobic regions of the world include rural USA, Central and South America, Africa, all of Asia, Eastern Europe, the Middle East, ... ok, yeah, the entire world is still homophobic except for dense cities in North America and Western Europe, and small pockets like Tel Aviv and Montevideo.