The point of this community isn't just "make fun of straight people", it's mocking toxic heteronormativity by reposting memes / comics / pictures of weird "haha i hate my wife" or similar memes, or in the case of this post "haha, if you have autonomy you clearly can't be married, because we're all trapped in our marriages".

Just like the original subreddit, hetero people are welcome here and hetero hate is not.

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.

Why would they?

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).