I bet "I have a girlfriend" would be equally effective

Try it and report back

I've told plenty of ladies I have a girlfriend, and they were still interested. Luckily for them, I'm gay and poly.

It discourages all the mostly-decent guys just fine, but the real assholes are just more motivated by it.

It absolutely isn't. That'll get the dude to get even more pushy and turn lesbophobic on you. Saying shit along the lines of you haven't had a proper man, which means: His cock. 😓

Or "perform your lesbianism in front of me for my titillation"

I have tried it several times and it really doesn't work quite as well. There is a group of men between those who respect my words and those who only respect my fictional boyfriend's claims that this approach is working on, but it's certainly less effective. Worst case you get hit with a "you just haven't been fucked by the right man".

Guys refusing to take a simple No for an answer always makes me think of Elizabeth's refusal of Mr Collins in Pride & Prejudice and how little some things have changed in the last 200 years.

Do not consider me now as an elegant female, intending to plague you, but as a rational creature, speaking the truth from her heart.

Including having to bring a man in (albeit the father, not a boyfriend):

[...] determined, if he persisted in considering her repeated refusals as flattering encouragement, to apply to her father, whose negative might be uttered in such a manner as must be decisive, and whose behaviour at least could not be mistaken for the affectation and coquetry of an elegant female.

I totally read this as a gender reversal thing, having had to say that to a few girls in my time.

I wonder about these things. Is it just coz I'm an enby that I don't see the gendered nature of things like rejection?

Guys can be shy, or pushy. Girls can be shy or pushy... Its not exactly the same but it's not exactly different either.

I've never interpreted this statement in this way. "I have a boyfriend" just seems simpler and more widely accepted because it doesn't imply a lack of interest. Insecure guys can interpret it as "I would love to but I'm not a cheater", whereas a simple no triggers a more personal rejection response.