Sure snood, but...
You have this view of why child marriage "seem to be wrong" that is shared by 99% of us in Modern Western culture. This view was not shared by the great majority cultures in African, Asian and North America.
It isn't just child marriage, there are a whole bunch of issues that all Modern Americans agree are absolutely true... even though most other cultures outside of those with shared Western European roots would disagree.
So answer me. Does Modern Western society have some access to absolute moral truth that other cultures didn't have? Given the history of Western culture dominating these other cultures, it seems a little uncomfortable to even ask this question (but that never seems to stop me).
Either we treat cultures as equal and allow for drastically different ideas of right and wrong, or we assume that indigenous societies "sanction rape" (however our culture defines "rape") are in fact inferior.
Imagine approaching a a member of a Native American, or Pre-Colonial African, or Asian culture
and telling them that the marriage customs at the center of their culture that they hold dear actually "sanction rape". This term would be completely meaningless to them. Of course, they had moral ideas that are likewise meaningless to us... either we treat these cultures as equals or we accept that we are their superiors.
Historically, of course, this actually happened. The fight against child marriage was at the center of the drive to Christianize Native Americans living in what is now known as California. The writing at the time talk about the "barbarism" of Native marriage customs that were so foreign to the European conquistadores. These Native Americans were converted and those that resisted were killed.
When you look at the crimes committed by absolutists, I am very comfortable being a relativist.