What do I do about a prescriptive GM/player who argues that gender and sexuality aren’t personality traits?

I play RPGs (usually D&D but other stuff too) every week with a group of friends who I’ve known since middle school. The group is me; Ecru, my best friend; Khaki, our usual GM; Desert, another friend; and Lemon, our resident problem. (Names changed to protect identities). All of us except for Lemon are part of the LGBTQIA+ community and proud of our identities, while Lemon is cisgender and straight.

Lemon wanted to GM a six session D&D5e adventure (it started as a two-shot and the time frame kept growing as he actually wrote the adventure) set in another part of our usual setting as a break from our main campaign, and we’ve been making characters on and off for a while. We’ve all made characters with diverse gender identities, sexual orientations, and romantic orientations before, and it’s never been a problem (in our main campaign, Ecru’s rogue is nonbinary and my sorcerer is awkwardly-not-straight-and-not-going-to-specify, and even in this adventure, Khaki’s character is gay and my bard’s entire motivation for being involved is to impress her girlfriend).

Last week, we were fleshing out a backstory and personality for Desert’s character, and Desert wanted the character to be bisexual and gender-questioning. That was fine, until Desert decided to put that as a personality trait for the character. Lemon was really upset at this and he started arguing that LGBTQ characters were fine, but that’s not a part of personality and so shouldn’t be written down as a personality trait.

I feel deeply uncomfortable with Lemon’s statement (as do others in the group). For me, as well as the rest of the group, our gender identity and sexual/romantic orientation has shaped our personalities, especially because it shapes how we interact with society and with ourselves. To me, it feels like he’s trying to be prescriptive and tell us what can or cannot shape our personality when he has not personally experienced any of what we have (for instance, I’m nonbinary at a bigoted school and have to be careful about even mentioning anything about my gender, which has made me more cautious than I was in middle school).

How do I deal with another player/GM being prescriptive about personality traits when he has not had the same real life experiences that the rest of us draw on when creating personality traits? I don’t want answers that say “that’s not how personality traits work” or that talk about RAW personality traits, as our group prefers to use custom personality traits to help us play the characters better rather than the suggested ones for our backgrounds and Desert says that it would be a useful personality trait; I also don’t want answers that suggest asking Lemon to leave the group since that’s not an option I can bring up to the group.