The first thing that comes to mind is that bi may be a bit of a vague label in the first place. Referring to
The KInsey Scale, do you see bisexual as all of 1-5, i.e. even people who are mostly attracted to one sex but who acknowledge a even a small attraction to the other sex? If so it makes it a very broad label that doesn't really pin much down. But then if you don't define it that way and, for example, definite it as Kinsey 2-4 do you then call Kinsey 1 straight and Kinsey 5 gay? Or do people with those attractions not get a label at all? I think when you're aware of an attraction to more than one sex you quickly become aware of that complexity in a way that people who are exclusively attracted to one sex do not need to be.
Then there is a question of what purpose a label serves. I think these are possible uses for a label:
1. To set your own expectations of what the future will hold.
2. When searching for a potential partner, to focus your attention on where you should be looking.
3. To manage the expectations of others.
So someone who is gay needs to be honest with himself that the wife and two kids life that all children imagine they will have when we grow up isn't going to happen. Adopting the label "gay" will sufrely help with that will possibly help him focus on places where other gay men are to be found and also provides a way of having a conversation with friends, parents etc. to set their expectations. All things that are commonly described as "coming out".
But what about the bi man who has fallen in love with a woman. He knows he also has an attraction to men but, at least for the time being, his lifestyle is indistinuishable from a straight man. There is the issue of being honest with himself but he doesn't need to reset his own expectations in quite the same way the gay man does. And if other people continue to treat him as if he was completely straight will that cause any problem or distress? Perhaps the label isn't so useful here. That's the situation that applies to me - my wife knows and so do some friends, but I don't go round waving a flag.
Then take the situation for my half brother who had a relationship with a woman, then came out as bi and started a relationship with another man. Some of those around him say he must really be gay and the first relationship must have been a sham. Who are we to see inside his head and say that what he says is not true?
Finally, a gay friend of mine with whom I have mutually mastubated was bending my ear about "bloody bi guys" who he said were happy to have sex with him but would not become his boyfriend because they couldn't give up also havings sex with women.
So to summarise, a label has to be more useful than a liability for people to want to adopt it.