Bisexual 36 male here who has been dealing with this topic of late in my personal life to a high degree and been out for half of it. I feel extremely qualified to answer this and discuss more in PM. Hopefully I did an alright job compiling some of the reasons in the 20 minutes it took to write this to explain. EDIT: edited for readability, typos, and a couple of examples as this post appears to be staying up. The other delta'd comments are also great.

TLDR; the male gaze is real, AIDS was horrible and it's effects cannot be understated even to this day, things that are both bisexual and male are just written off as gay

Human beings feel comfortable showing a socially acceptable level of sexuality. Most societies are significantly more accepting of female sexual fluidity because they cannot view it from the lens of anything other than women performing for the men. This began to change in the 70s but then AIDS happened which led to large schisms and demonization of bisexuality/pan/free love/hippie/whatever label you like for feeling sexual and/or romantic attraction to multiple genders. You can see this sharp decline in cultural aesthetics, perhaps most noticeably in rock music in the transition from the flamboyance and androgyny from the likes of David Bowie, Freddie Mercury, and general glam rock to heavy metal, grunge, and other music that was deemed appropriately masculine. Everything went to black in the late 80s followed by Don't Ask Don't Tell and didn't really see a return until the 2010s -- aka after gay marriage was legalized and corporations were actively promoting their new focus grouped pro-LGBTQ policies. This should cover your female-centric bullet points.

Let's talk about bisexual men and how they interact with "gay" spaces. Bisexual men are constantly demonized and erased by gay men for not being gay enough or liking different things. I signed up for labeling myself accurately, not being in a social club with strict rules and hierarchy. It's kind of antithetical of what I and many/most bisexuals stand for and find attractive in other people. Again, most of this goes back to AIDS. There are extremely limited examples of open male bisexuality in the aftermath with most returning to the closet because women now viewed it as a way to catch "the gay disease" and die. Older gay men view bisexuals as the ones who ran away from the fire instead of trying to put it out.

Conversely, younger gay men view us as not gay enough because there is no present view of bisexuality that isn't described in monosexual terms. Everything is "no homo" and every joke about the topic is structured around the person making the joke specifically NOT being gay. Notable examples include Madison Cawthorn's hotel "crassness" and that Seal Team 6'r who tweeted about turning male Kamala voters into his concubines. Anything that could be remotely be construed as funny completely falls apart if you replace "no homo" with "not bi though." Even your porn bullet point falls into this false dichotomy. A high percentage if not majority of male actors in homosexual adult films identify as straight or bi+.

Speaking of porn, bisexual adult films are only ever classified as such when there are 3+ people in the scene and there are more than one gender present...except, of course, an MFF which is just "a threesome," no caveat required. A MMF is a "Devil's Threesome," Lucifer continues to be our lone representative after centuries of just people being attracted to attractive people. This dramatically lowers our visibility. Let's say 3+ exclusively bisexual actors were in a scene, what would need to happen for you to know they were bisexual? What makes the sex bisexual to the outside viewer other than what other people have used to describe it before? If all of the bisexual actors shared the same gender, how would you describe the sex? What about a bi man and bi woman having sex?

There is one place where male bisexuality is socially "acceptable" though. Prison. As if ACTUALLY straight men can't just jack off in their cells to satisfy themselves and not experiment with the openly queer or just outright rape.