I mean, they’re saying they think you are the same gender as them, and that they aren’t interested, in a not very polite way.

I don’t see how thats … even implying anything.

‘Sorry, I’m straight’ is a pretty direct statement of assuming or asserting your gender, and also saying they’re not sexually/romantically interested in you.

I’m basically a bi dude, trans inclusive, but I’m not gonna lie, I don’t tend to find myself attracted to many… agender/ambigender/genderfluid folks. I’m not like, against dating or fooling around with such people on principle or anything like that, its just that I rarely find people who identify as such, that I consider attractive.

That being said, I wouldn’t turn someone down, or accept a flirt, via assuming their gender. I’d use a bit less discriminatory/categorizing phrasing, probably involving the phrase ‘my type’, which is gloriously vague.

But… I’ve been to a good deal of bars, including gay bars, lesbian bars, bars with mostly straight folks, bars with mostly not, bars with a decent mix.

I have, many times, seen people that I know are gay, or lesbian, or bi, just lie to people they’re not attracted to, and say ‘Sorry, I’m straight.’

They know they aren’t straight.

They’re just lying, perhaps plausibly lying to that person who doesn’t know them, to get that person to go away.

In both your case and my example cases… there’s really no need to infer or guess that the person using this phrase is being fairly rude, and a lot of that is intentional.

I’ve even seen straight people say ‘Sorry, I’m gay/lesbian’ to get out of a heterosexual flirt from someone they’re not attracted to.