Because the cab driver isn't going to say "Oh, you have kids? I bet you hate dressing up as robotic ninjas" forcing you to have to correct them and explain that

Because the cab driver isn't going to say "Oh, you have kids? I bet you hate dressing up as robotic ninjas" forcing you to have to correct them and explain that, no, you don't fit his presumptions. It's not the cab drivers fault necessarily, people are socialized to expect that other people are straight, often because many people hide it. Hence the circle. By pointing out that I'm gay, when NO ONE (literally, I mean no one, most people don't believe me) expects it, I break that assumption and cause that person to second guess that assumption in the future.