A bi question

I only realized this a few years ago but why do some people that are non and surprisingly also part of LGBTQ+ hate bisexuals? I'm genuinely confused because why chose which gender you are attracted to if you can have both? (more people for you that's a win win for me) I don't hear people hating on pansexuals but I keep seeing things like 'you can't be a bisexual, you're either straight or gay.' I heard somewhere that they hate them bc bi's believe that there's only 2 genders (how the heck did they conclude that?)

I remember when DC canonically change Tim Drake's (3rd Robin) sexuality as bisexual and they change his girlfriend with a boy and it made me '???' you know you didn't have to change his girlfriend to a guy for people to know he's bi or part of the lgbtq+. That also applies in shipping fandoms like 'he's not bi because she/he's dating the opposite gender. That means he's straight!' It's ok to have your hc but some people do be hating on people having a hc/oc that the character is bi but dates the opposite gender. I'm not really a person who engages deeply in communities/fandoms but I do see some of them sometimes. I'm curious why and sorry if it sounds rude(?)