This is completely anecdotal, so make of it what you will, but back in the 00's, everyone and their dog were bisexual for asspats, at least in my scene. So for me, that's where my general disregard for bisexuals comes from.
Also, this might be related to their politics as well, but it's all "bi erasure this, bi erasure that." These people feel safe coming out as bi because gays and lesbians stood up and took the beatings. There is a tendency for bi's to claim ownership of the gay and lesbian movement (kinda like troons and feminism. Or like troons and Stonewall. Or like troons and pretty much anything, but I digress). That's why I (probably) wouldn't include them in my kind of gay activism, because it muddies the messages somewhat.
That being said, I've had sex with bi men and don't discount them as sexual partners, though I think I would find it difficult to have a romantic relationship with a bi person. I've often wished that I was at least bisexual, because at the end of the day, having a long-term relationship in the heterosexual fashion is probably easier in terms of fitting into society at large. But I tried the woman-sex-thing and it's not for me.
And finally, I do understand that bisexuals also wouldn't want romance and sex with most trans-people. I have tried to understand bisexuality and my impression (from the one or two guys who I know that are actually bi) is that they tend to go for men that look like men and women that look like women, not plastic surgery disasters dug up from the Troon Lagoon. I've also noticed it being a bit of a meme with the gc crowd lately, and they should probably stop it.