There's nothing people love more than giving someone a good ol' fashioned "I told you so". Especially when it comes to whom we date. I'm guilty of it too. When my best friend from high school went back to dating the woman who previously broke his heart, only for their relationship to once again end in shambles, I gave him a big fat I told you so. When my relationship with my last ex didn't work out, which a number of my friends predicted, they all insisted on rubbing it in my face.
It's human nature to some folks. They find few things in the world more satisfying than being right (and letting someone else know they were wrong).
But this natural, albeit smug, tendency goes even further when it comes to bisexuality. There's an institutional glee when someone who's bi, especially a bi man, later comes out as gay. There are seemingly few things the LGBTI and straight media love more.
Let's take the Olympic Diver and heartthrob Tom Daley. Back in December of 2013, Tom Daley came out on YouTube saying he was dating a man, but clarified, "Of course I still fancy girls." This, needless to say, led folks to believe that Tom is bi. Many folks, however, questioned his bisexuality in the media, saying that he is surely later to come out as gay. In fact, it led to a large debate on the existence of bisexuality in men.
Then, five months after saying he's attracted to men and women, was on the talk show, ITV2's Celebrity Juice, where he was grilled about his sexuality. He was asked by the host, "Let's get down to the crunch. You're a gay man now." Daley responded with some awkward variation of "I uh", or "I am." Having rewatched the video repeatedly, I can safely say I'm not sure which one it is. But it is clear that the words, "I am a gay man now" did not leave his mouth, despite the fact that's how multiple news outlets reported it. One went as far to have the title read, Tom Daley: "I'm definitely gay not bisexual."
Only a few smaller outlets made it clear that those were not the words Daley used. The media was putting words in his mouth. Just a few weeks before Daley appeared on the talk show, the New York Times embarked on "The Scientific Quest to Prove Bisexuality Exists", quoting the gay writer Andrew Sullivan who predicted that Daley will come out as a gay man because many younger men in their teens and early twenties use the bi label as a stepping-stone on the way to "full-blown" gay.
This is true, many gay men do use the label in this transitory manner; many bi men don't. If we left it at this, I think we'd be all set. I don't think there's anything biphobic admitting that some men do use the label as a transitionary step.
However, and this is when things start to get awry; it's seldom left at this statement. This statement often fuels (usually gay) men to then not believe a man who comes out as bi, as either they, or a myriad of other gay men they know, used the label as a stepping-stone. So these gay men decide that the best way to respond to this young man who just came out as bi is to offer a pedantic sentiment, saying something along the lines of, "Oh honey. I was bi too. You'll get there like the rest of us."
It's statements like this that are problematic. First off, you're just being an asshole. No reason to sugarcoat it. You're being condescending and presumptuous. Second, you're being egocentric. You're assuming that just because you used the label on the way to gaytown means that every younger bi man is. This is unequivocally false. Everyone's sexual identity and attractions manifest themselves in various ways. Third, you're engaging in what psychologists call the availability heuristic. It's a faulty mental shortcut that relies on immediate examples that come to a person's mind when evaluating something larger. In other words, you can only think of the one guy who came out as bi and then gay so you assume all bi men do that.
Fourth, you just really want to be right. Herein lies the crux of the issue. If you didn't want to be right you wouldn't say anything. If you claim you want to be helpful, by telling the recently coming out bi man that it's okay to be gay too, if he so chooses, then actually say that.
I think it would be okay to respond to a man who comes out as bi with, "That's awesome! And if later down the line you find yourself more attracted to men or women that's awesome too. As long as you're happy!" That would be an example of statement that may actually be helpful if you feel you must say something.
FYI — Not once in my years of coming out as bi to gay men have I received a response like this.
It's also fine to note that another completely reasonable option is to say, "That's awesome! Congrats honey."
If you don't have this desire to be right, or the larger institutional glee, then you really don't need to say a thing. And if he does come out as gay later on, after you initially said nothing, you can then say, "Awesome!" Or screw it, if at that point you want to give him a big fat I told you so, fine! Do it! You can tell him that you thought he was gay all along and have a laugh about it.
But let's say something else happens. You tell the bi man that he will inevitably come out as gay, but it turns out you are dead wrong. He identifies as bi until the day he dies. What you did was then really frustrating, invalidating, and damaging. A fellow queer came out to you and you dismissed his identity. Imagine if you came out as gay to a fellow queer, and he responded with, "Nahhhh, you're not gay." That would really hurt.
So instead of having this smug satisfaction that comes from correcting another person's sexual identity, let's be supportive. Let's believe. And if believing is too much, then let's keep our mouths shut. Because when you get rid of the petty "I told you so," you quickly realize you can only hurt someone by dismissing their coming out process and sexual identity.