Let's review.
Legally presumed. So, that is your only objection? If the law said they were able to consent, then that's ok, right?
The question cannot be more clear.
Sure, if we as a society decided that 16 year olds could consent to starring in porn, then that would be the law and I would have to live with it, though I would certainly not be in favor of such a change
We aren't okay with that, of course. So I'm not sure what your point is.