You could argue that the sexual exploitation of children goes on all around us, and so adding a little bit more won't change anything, but by doing so in a way that demonstrates the message of how harmful it is, might be a net benefit if it causes people to see the problem and do more to stop it. In other words, intention matters.

My personal opinion is that I agree with you, and I encourage all children with confidence to express themselves sexually for their own and everyone's good. But I don't think you've single-handedly dismantled the opposition's argument here.