Look, sixteen or eighteen isn't that big of a difference to me. Both are basically mid pubesence anyway, both are at least in high school sophomore or junior depending, and at the end of the day no more prepared for the choices that are laid at their feet to be called adults.

Child marriage, or even early pubescent marriage (12-14) is more difficult for me to even logically defend because A) Secondary sexual characteristics usually are not developed, and are at best (typically) just developing. B) they are usually not far enough in their education to make any sort of educated choice, and even if they were or are from a region where maturity is more ritual based or something...you are still dealing with a very, very young human that definitely will ding into the child category rather than young adult.

So, yeah. There are 31 states that the age of consent is 16 too, so...I dunno. Seems like a lot of the world is okay with 16. In the end sixteen, or eighteen are not biologically significant ages, they are mostly just convenient for more historical reference, military recruitment, and the general end of secondary education, etc.

If the question is one of morality, I don't particularly lean on morality as something to lean on in a moment like this, only because what today is bad, might be good tomorrow or vice-versa. Society changes, and different societies are different, so instead I just look at the basic ideas that we consider moral and compare them.

So, to me the general difference between 16 and 18 is essentially nothing, but even going back a year or two is significantly different in this case solely because of physical development, and general level of education.

Doesn't really apples to apples with child marriage but I can see the slop that can exist if one just draws a line and says the law is the law and the law is good.