What does anal sex feel like?

Like taking a big poop, but it goes backwards

like a poop that confuses you because it is neverending
like there is the expectation and partial experience of relief but it is stressed out all the time
eventually your mind breaks down from feedback loop and it turns into a comfy, brainless massage

My girlfriend says it feels like she has to take a shit, but it's more taboo is why it's arousing to her