Because she is using his sexuality for her own gain, this also contributes to the idea of gay men being used as an accessory by heterosexual women. She sees gayness as the “actual reason” he wouldn’t be interested in her. This is because she is only using homosexuality, or the idea of it, as a way to make herself feel better and legitimize her rejection.
At the very least, I think it’s fair to say that Eilish’s lyrics trivialize gay men’s sexuality. Some fans have responded very negatively to this meaning, even calling it a fetishization of gay men.