However, as you note, age of consent is a very important factor. The two families could be enlightened, mature and responsible, but if either kid is below the age of consent, then both kids are at risk of being registered sex offenders, and the enabling parents may be in for legal trouble also.
I would prefer to take the Netherland's approach, but the legal issue trumps everything, in my opinion, in these hyper-puritanical United States.
You're right...these laws were likely written during a more conservative age, but even though attitudes have changed, it is difficult to change the laws. Just do a search for "Blue Laws" to see plenty of examples of this. I think that it's quite possible that now the situation is reversed, that the laws are influencing attitudes. Just look at the drinking age. I'm sure there are plenty of parents who don't give their kids wine simply because the drinking age is 21.