Why not? Do you think it's good for unmarried 18- and 16-year-olds to be having sex, making babies, and spreading disease? What about 15- and 14-year-olds? 13?
We're talking about something that's wrong and has significant bad consequences, both for the participants and for society. Some kind of "get in trouble" - a fine, community service - might make some of them think a little.
I should have qualified by saying “legal trouble”. Legal and moral are different animals.
From what I’ve personally observed, a lot of people lose their virginity by 16 (or claim to) and almost everyone has by 20. The vast majority of boys are eager to “lose it”. For me, it happened at 17, but would have done it earlier had the opportunity presented itself. Girls are another issue. Is that a sexist double standard? Absolutely, but the sexes are obviously different.
That being said, as an adult, I would never encourage anyone under 21 to go out and have sex. But it is stupid for a young man to be labeled a sex offender because he had relations with someone 2, 3 or even 4 years younger than he was.