Two arguments I can think of:

The first is that you're banning free expression, and banning free expression is inherently harmful.

There's also the displacement theory - with the legal content being much more accessible and regulated to ensure minors aren't involved in production, it displaces illegal content that does harm minors.

I don't think the theory behind "free expression" covers things such as CSAM or incest porn. It's ok to say that content that aim to excite the viewer about minor abuse should be forbidden.