Depictions of what would be illegal in real life in porn are generally considered illegal and obscene in the US.
Same with even fantasy IRL porn, hence the endless use of "step-" relatives and not real relatives. Or generic "teen" instead of having an adult actress just claim they are underage.
Just because nobody is currently going out of their way to police something and arrest people doesn't mean it isn't illegal. All the more legit porn sites scrubbed a ton of their material a couple years back because US lawmakers threatened them.