comparing apples to apples

I have little doubt there are some cases in which child marriage does occur in practice as de facto child sex slavery

And I have little doubt there are some cases in which marriage between adults does occur in practice as de facto sex slavery.

I am reminded of the anti-polygamy activists who insist that polygamy should be strenuously opposed because some polygamists abuse their wives, as if monogamous relationships are never abusive.

Any institution or practice should be judged on its general performance, not on the most abusive situation possible. Naturally, we need to be aware of such abuses in order to reduce future abuse, but as long as there are people we will not get rid of abuse entirely, and if we refuse to recognize the differences between individuals and situations we risk becoming the sort of abusers we sought to oppose.