The things that were determined right or wrong by God were for the good of mankind. Such as adultery - the damage it causes the family.
Those that don't accept a God, may try all forms of relativism but they still must come back to the respect of the individual. What one individual does effects another. When one individual stabs another because there is no harm - that other individual shows this is wrong because he has a stab wound.
When a man seeks child sex and claimns no harm - we have the child and the child's future life as the result of his ten minute sexual pleasure. It is abuse and endangerment of that child for his own selfish pleasure. No man should have the right to endanger another for his own pleasure.
Now the claims that there is no harm will have to be fought as the lies they are. What about all the people coming forward with claims of harm by the priests? Are they different than other children? There is no proof that they selfish actions have no effect on an innocent child. There is no proof that it will not cause them to seek the same sexual gratification when they are adults. This in itself is an abuse because then no longer is the person heterosexual - he is now a pervert seeking child sex same as the prior generation. This very fact is an abuse. The first pervert now has taken away normal sexual development ffrom this child and begat another sexual pervert as himself. This should be against the law.
But that's just the problem. They don't have to come back to the respect of the individual. Quite often both the oppressors and the oppressed simply accept their situation as being the natural order of things. Whole empires have been built on that basis.
(Religious people certainly are as prone to such impulses as non-religious ones -- we simply have a reason not to do something, even if we ultimately end up breaking the rules.)
Ultimately I see this as a matter for Evangelism, and treat it as such.