My ex turned out to be gay. After 18 years together, he first said he was bi, then 'probably just gay', then he retracted that last statement and said he wanted a divorce because I was impossible to live with.
He's now in a relationship with another man, and I also have a new boyfriend. This new relationship has been a revelation - whatever ex thinks, I am convinced he's gay. The difference between my relationship with him and my new, straight boyfriend is so marked. I don't know now how I ever thought my ex actually loved me and fancied me.
I'm in a support group for people in my situation, and it's very common for men to come out to their female partners as bi before going on to say they're actually just gay. A few are genuinely bi, but on the whole there's a bi 'phase' and they eventually come out as gay.
I really don't think that helps. It's more common than most people realise. During my ex's bi phase, I spent a lot of time on the bi boards of reddit, trying to understand and be supportive. There seems to be a very wide variety of ways that bi people identify, and I think that makes it harder for straight or gay people to understand.
Ultimately, I believe that sexuality is highly individual.
However, given my personal experience (finding out my partner of almost two decades was actually gay was/still is a massive trauma that caused huge destruction in my life) and the very similar experiences of other straight partners who find their way to the support group I'm in, I would be very wary of any man who says he's bi. I don't like that, because I don't want to be suspicious and I don't like how phobic it seems, but I am.
It is so, so important to eradicate prejudice. Everyone should feel free to be their true self. I know that relationships end for all sorts of reasons, but I feel robbed of the opportunity to have a lifelong partnership because my ex was too afraid to be gay.
Gone off at a bit of a tangent there, sorry.