I think your question asks more about the male mindset than about bisexuality.
I want to say first of all that I am not a man-basher. I LOVE men. To a fault, even. They're amazing and I could NOT live without them. Seriously. And I know a lot of people assume someone who is bi is equally attracted to both genders. I am NOT. I lean very heavily toward the male side as far as preference goes. Men are great.
Most men (I'm generalizing) seem to have very close-minded views on sexuality, whether it be hetero, homo, or bi. Most men assume a man is either gay or straight, and assume a woman is bi. This is why men seem to be shocked and totally disbelieving when women aren't interested in MFF threesomes. It is assumed we are all into women, maybe because women (generalizing again) have no problem with openly expressing affection (if we hug another woman, obviously we want to have sex with her, right?).
I think that as much as I respect and love and adore men, I see them (GENERALLY and in reference to hetero men) as very narrow-minded, and usually extremely resistant to even considering that not all women are bisexual or are even willing to go there "just to try it".
Also, I know the assumption most people have (and woman think this too) is that men CAN'T be bisexual. It is assumed that if a man has sex with another man, he HAS to be gay. That the sex he also has with women is fake.
I think it comes down to whether or not you choose to believe the "popular" opinion (which, in my opinion, is usually wrong) or explore the issue and look for facts.
The fact is, both men and women can be bisexual. Both men and women can be homosexual. Both men and women can be heterosexual. And I don't think that one or two experimental encounters determines sexual orientation. If it did, almost every single straight person EVER would be gay or bisexual and almost every single gay person EVER would be straight.