I’m approximately cishet, and I approve of this joke, for whatever that’s worth.

I agree that it wouldn’t be okay for cishet people to make this joke about gay people. For one thing, that would be punching down at the systemically oppressed, rather than punching up, and that matters.

Married people can still just decide to do things. This looks like a low key “marriage traps you” sentiment, which is an unfortunately common mindset for heteros (particularly older generations).