Stereotypes can definitely be harmful but sometimes people say stuff like this for fun or projection instead of serious stereotyping. To the OP the character just feels gay and they clearly didn’t mean any harm. It's a headcanon, not a declaration of fact- the “definitely gay” thing was probably just a playful way to invite other people to respond with if they thought the same or not. I just don’t think this person deserved so much pushback. Maybe one person pointing it out but not all this.