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Here in Utah, if she is 15 and he is 40 and they have sex, he'll be locked up and they'll throw away the key.
No explanation is necessary, such activities being sick and perverse, etc.
Of course, if the 40 year old man wants to marry the 15 year old girl, then circumstances change. It's no longer sick, perverse, etc. It then becomes perfectly legal and has the blessings of the same state that thought it a crime just a paragraph or two ago.
Clear?