Some girls get their period at 10 or younger is it okay to bang them? of course not , while it may be ' natural ' it not moral .
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Define morality. According to Kant, in his "Critique Of Pure Reason", morality is an illusion derived from the paradoxical nature os altrusim and selfishness in Society. According to Nietzsche, morality is an illusion created by the weak to subdue the strong into doing their bidding. There has never been any universally accepted standard of morality. In some societies, girls of the age you mentioned not only have sex, but are sold by their families into marriage by men old enough to be their grandfathers. Are they immoral? Maybe to you, but morality is a Human rather than exact science, so what is wrong for you insnot wrong for everyone else.

SUCKMYMUSCLE
P.S: And BTW, very few girls get their periods at or before age 10. The average age of menarch in the West is around age 12.