Well, Marek, I don't think that anyone would let a child get a body mod

So your argument is a bit off.
We are all talking about adult individuals here.
We are lucky enough to live in a world where we get to decide if we are going to change or decorate our bodies and in what way. We can choose to dye our hair red, make our nose smaller, pierce our eyebrows or do massive scarification or to do none of that and stay as we are.
Importance of having such choice is of utmost importance and banning some aspects of that choice is a gateway to restricting our other freedoms. Who are we to say when it is ok for a girl to have breast implants? So what if she only wants bigger boobs or if she had a mastectomy? How is that our business?
Who decides what gets banned? Do we get to, globally vote on it? (I don't think so)... What is next to be banned than? Piercings? Wearing jeans? Will we get appointed appropriate hair color like in North Korea?
When we see something extreme we don't approve of, and start demanding restrictions as a selfish shits we all are (human nature is "me first") we should all think of the consequences of such demands. And first off : Is it my business what someone does in their own time, with themselves and to themselves?