Fellow bi/queer here. As a person who isn’t homosexual I might have a different perspective on this (and I’m very open to changing my perspective), but I would agree that you should try not to let this bother you, because it’s really not about you and isn’t a denial of your love.
The nuance of queer sexuality is something that a lot of straight people (especially straight people like your boyfriend’s parents) don’t really understand or frankly care about (and it can be exhausting to try to explain). To them, “queer” is interchangeable with “gay.” So I think your boyfriend is choosing to use the simplest language he can with them and also protecting his energy in a process that is already draining.
I also know a lot of queer people who use “gay” as an umbrella term. If someone on the street asked me my sexuality, I wouldn’t say “gay,” but I’ve been called “gay” in WLW/sapphic circles. If I’m in a group with other queer women and lesbians there are lots of passing references and jokes about how we’re “gay,” etc.