Story time:
When I was in high school, a guy in one of my classes was the kind of guy who'd use the word "gay" in that manner. I don't think he was actually explicitly homophobic, more just had an edgy teenage sense of humor.

One day, he was talking with a friend of mine that he didn't know was homosexual, and at one point he "jokingly" said to my homosexual friend, in that exact same joking way: "What are you, gay?"

You could hear a pin drop. Everyone in the room was uncomfortable after that. If he said that to anyone else, you'd just brush it off as an ignorant joke, but because of who he said it to, it became targeted, even if that wasn't his intention at the time.

I had never met a gay person and have never had (and still dont have)

Roughly 5-10% of people are gay. You have almost certainly talked with many gay people, you may have even made these kinds of jokes in front of those people. This might be a good time to reflect on why those people might have chosen not to share their sexuality with you.

Either way, I'm glad we can have an open conversation about this here - I think most people have gone through some kind of "problematic" phase at one time or another, and openly talking about it with others can help.

Extra context: I'm straight, most of my friends aren't