I've seen your comments on how you don't think biphobia exists, and I disagree there, but I do agree that most straight and bi women view bi men as 'tainted'. Really if you drill down far enough it's rooted in patriarchy and the almighty penis. Straight men slut shame women for the same reason straight women shame bisexual men, they're both 'tainted' by dick because sex involving dick is the only type of sex that matters to a lot of people. It's why straight men don't see lesbians as 'tainted' in the same manner despite the fact that they have sex with other women. That, and the whole '1 girl = hot, 2 girls = twice as hot' mentality lesbian obsessed straight men fall into.

A woman, any woman: bisexual or straight, would laugh in his face if he even thought he had the smallest potential to be with a woman after admitting to being with men and being proud of his bisexuality.

Exactly. That's biphobia. It would be homophobia if he was being shamed solely for his attraction to men. But it's the ridicule that would happen after attempting to be with a woman again after admitting his attraction to men that makes it different from homophobia. What's happening is something very specific to bisexuals when it comes to intimacy. Straight women's reasons for thinking bi men are gross may be rooted in homophobia and patriarchy, but overall it's broader than that. People look at bisexuals and think we're greedy and gross for even thinking about being in a hetero relationship after being 'tainted' by the gay, or vice versa for same sex relationships after being 'tainted' by the hetero. Straight/bi women would laugh in his face because the assumption is that once you go in one direction you can never go back the other way. It's the erasure of bisexuality, and that's also a component of biphobia.