Because in Florida [url=http://www.huffingtonpost.com/2013/05/19/kaitlyn-hunt-florida-teen-felony-same-sex_n_3302713.html]you can be arrested and be charged with "lewd and lascivious battery", and be kicked out of your school[/url] it.
What terrible parents.
Edit: The pair were dating since the beginning of the school year at which time the girl who was arrested was 17. All parents were aware they were dating. The parents of the other girl pressed charges once she turned 18.
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Depends upon the country really and customs. In the UK the age of consent is 16, and if the person had only just turned 18 and the 15 yr old was soon going to be 16 then it wouldn't be that bad. Like a 15,11 month yr old and 18, 1/2 month yr old. That's not as big an age gap as first thought.