No, people don't reject child marriages just because they don't like it, it is medically known to be harmful and children can't consent.

Further, moral relativism is a dangerous and absurd position to hold. It might be convenient in excusing something you hold that's controversial but it also means a group of people can use it to defend any position at all they have. Any familiarity with what we humans have done and do should tell you this won't be about excusing disagreements over irrelevant things like ice-cream flavors.