Because our society is obsessed with binaries. If you've ever been attracted to someone of the same gender, you're thought of as "other". People (both straight and gay) assume that non-straight = gay. It's easy to see how misogyny comes into play: bi women are seen as "experimenting" before settling on a man, and bi men are seen as "easing their way out" before settling on being fully gay. It's really flawed, moronic thinking when you dissect it all. Like... how hard is it to believe that someone is just attracted to both girls and guys
The stupidest logic behind not believing bisexuality is a valid identity is "they always end up picking a side when they marry someone". Well... no sh!t they do, but that doesn't invalidate or discount the fact that they're attracted to other gender(s) as well. That doesn't mean their attraction to the gender that isn't their spouse's was only temporary or a phase, it just means they settled on someone that isn't that gender.
Some gays are jealous of bi guys because they "have the option of living a normal life". This logic implies that sexuality is a choice, and that it's impossible for them to fall in love with a woman because ew vaginas. In short, it's a combination of misogyny and ignorance toward non-binary thinking.
Many are misogynistic, racist, and even outright dismissive of bisexual and trans members of their own community. Basically if you aren't a 100% white gay, you're on your own for the most part. I imagine it has a lot to do with internalized self-hate. It's sad tbh. 
