Long story short, a lot of heterosexuals get uncomfortable/insecure around bisexuals. Meanwhile, a lot of homosexuals see bisexuals as traitors or wishy-washy for not picking a lane.
It's stupid on all accounts, but that's people for you.
{Pardon the buzzwords.}
Part of the issue there though is that many Bisexuals are Bi
sexual but not Biromantic. There's probably a lot of people who could categorize themselves as being sexually attracted to either gender, and would have
sex with either/or, but when it comes to have an actual full romantic relationship or wanting to have a life with said person of the same sex as themselves they absolutely don't want that or can't see themselves doing that or they try it and it just doesn't work out with the same sex. True Homosexuals are more like Homosexual and Homoromantic. They're not just attracted to the same sex in a sexual manner but they peruse relationships with people of the same sex because common secondary traits relating to socialization, body language, behaviors, fashion preferences, hobbies, how they interact with the world, that you can't just mimic, etc. are also part of what attracts them.
Sexuality is much easier to be fluid with than actual partnerships between people. Sex is sex, it can be done for pleasure and one off, making a life with someone needs more than just carnal and sexual attraction- although those things are also important to the partnership- because you're putting in more work, more commitment, and trying to have a comfortable long term life with the person.
Part of the reason trans people repulse others is not just because they're sexually disgusting but because they're acting and you can tell. They're doing an approximation of what they think the opposite sex should be. For all their claims of
always knowing they really don't. Sex/Gender isn't just something made up. There's aspects too it that are inherent, that aren't related to socialization or any other category. There's part of it that you only have, that other humans recognize, that only exists because you were born that sex/gender. It's simply not something you micmic or have surgery to make. Being Trans is a farce. Even the mythological
true trans. It's a mental illness being coddled by people who don't even believe what they're doing works, but they say all the accepting words and take the money of people who only get worse- not better. It's why so many troons end up old and alone, with a partner whose mostly there out of pity, or with other troons.
You'll see it a lot in de-transitioners too, that when they finally stop playing a part and be themselves- they can often find themselves someone who'll be in a relationship with them- because they've finally given up on faking what they think they should be.