There absolutely are red flags. For example, IF I was dating and my date treated the staff like crap, that would be it. No second date. Regardless of whether I was dating a man, a woman, or an enby. (I'm happily married, so this won't come up any time soon, but...)

At the same time, if you are simply not attracted to somebody, then an instant no, on either part, is completely fair. Men get entirely too hurt by the idea that a woman might simply not want to have sex with them because they aren't her type, aren't even the right gender, and this results in women being afraid to say no, which causes major problems later.

Instant rejection is about attraction, not prejudice.

But a lot of men who get rejected proceed to immediately prove the woman was right to reject them.