Kissing and hugging. Controlling themselves though. Maybe heavy petting. Maybe be celibate. I mean they are Blessed, but that doesn't mean they have to rush into it all at once.
Sex is wonderful, but a baby before one is ready in all ways to be a parent that can easily be hell.
Sex is the most beautiful gift God gave to men and women. However, I do believe that we are doing a good job preparing our children for marriage. As you say, marriage is difficult. Again I would echo hard work. Of course the most powerful way to teach them is by our own example.