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Why are there age limits? why is it illegal to marry a 12-year old? Helen of Troy was 12. Juliet and Cleopatra were still teenagers when they became famous. Most heroines of classic novels and poems were underage by today's laws. Thomas Edison married a 16-year-old. Medical studies show that the best age for a woman to have children is between 15 and 25 (lowest chances of miscarriage, of birth defects and, last but not least, of the woman dying while giving birth); while the worst age is after the mid 30s. And the younger you are, the more likely you are to cement a real friendship with your children; the older you are, the more likely that the "generational gap" will hurt your children's psychology. Therefore it is much more natural to have a child at 16 than at 40. In countless countries of the world women have their first child at a very young age, and stop having children at a relatively young age. Nonetheless, in the USA it is illegal to have sex before 18 (but, note, only if the partner is over 18, which is like saying that it is ok to rob a bank if you are a banker), while it is perfectly legal to get pregnant at 40 or (thanks to medical progress) even at 70.
Note that all of these forms of marriage were common in ancient times before the introduction of Christian "morality" (Islam, which came after Christianity, still allows them). Therefore one could find stronger arguments in favor of these forms of marriage than in favor of homosexual marriage (which has been relatively rare throughout the centuries and is persecuted in just about every society of the world).
Today incest is illegal, but it was common in ancient times (all kings and queens of Egypt married their siblings or mothers): why is it illegal for two siblings to get married and why does incest disqualify a couple from adopting children?