(I’m reading a book in the library when a random guy approaches me.)

Guy: “Hey, babe. Wanna get laid tonight?”

Me: “Sorry, bro, but I’m a lesbian. And I highly doubt even a straight girl would accept such an offer from a stranger, really.”

Guy: “You’re a lesbian? That doesn’t matter. We can still hook up! You can bring your girlfriend!”

Me: “I don’t have one. And being a lesbian means that you only like girls, not guys.”

Guy: “Then I’m a lesbian, too!”

Me: “No, you have to be a girl.”

Guy: “Does that mean my girlfriend’s a lesbian?”

Me: “If she’s dating you, then no. Wait, you have a girlfriend?”

(At this point, an embarrassed-looking girl comes over to me, apologizes, and drags him away. The next day, she’s again at the library, and I learn that she’s bisexual and has now dumped the rude guy. She is now my girlfriend!)