If you are a man and have sex with men and identify as straight then you are doing it wrong. That's not what straight means.
There's truly absolutely nothing wrong with bisexual. If you have sexual thoughts towards other men then you are bisexual/gay. Deal with it.
Semantic arguments like this are always difficult, but I'll try to explain it as I understand it. This may not be "correct", but it's how I've heard it explained before and makes sense to me.
There's a distinction between what gender(s) you fall in love with and what gender(s) you lust for. The scenario given in the OP is about "straight" men who have homosexual sex. This is contradictory right? However, these men who identify as straight probably only feel like they can fall in love with females. That's why they identify as straight. As far as sex with other men goes, that's purely lust. They're satisfying a particular primal urge to fuck, that they may not be able to fulfil with females.
However, a man that can fall in love with AND lusts for both genders would probably identify as bisexual.
And a homosexual man probably only loves and lusts for other men.
It's semantically tricky, but the main thing is understanding the difference between love and lust.
Then you're really not trying...I can't come to with a more close minded claim than that one.