i have several theories
1. Psychological Gameplay Effect Theory
The only "female" enemies (mindflayer, mannequin) are annoying as shit, creating a poor image of women in the (mostly male) fanbase, and elevating respected (male) bosses (Gabe, Minos) out of how fun their fights are (kinda gay fr). (NOTE: This may explain the "this game made me gay" steam reviews)
2. V.A. Theory
Gabe and Minos (and Sisyphus but he has a dad bod, while G%M are BUILT) have really good voice actors, and speak in powerful tones of voice (hot) meaning people are more likely to be unreasonably(?) attracted to them.
3. OST Theory
ULTRAKILL's OST is mainly breakcore and similar genres, and for some reason (not under investigation because i don't give a shit) people who like breakcore always be gay as hell.
i lied i only had (:)3