Also on a separate note, a lot of people are using the wrong pronouns for OP's girlfriend. Just because a nonbinary person uses traditionally gendered terms (ie, boyfriend or girlfriend) doesn't mean you can use the wrong pronouns. The girlfriend uses they/them. Respect that.

The B 100% stands for bisexual, your girlfriend is a biphobe and I'm so sorry you have to deal with that, OP. You can either take the time and energy to try and educate them (which is not a necessity, because that's no one's job but their own) or you can leave them, because it shows that they don't respect your identity. You are allowed to call yourself gay, if you so choose. If you want to identify with the community, then you are allowed to. It is your identity, your label(s).

I went on a rant, sorry, but tldr; your partner is wrong, on so many levels.

*they. OP says the girlfriend's pronouns in the post.

OPs girlfriend uses they/them, it's not hard to respect.