"If you knew that every time you got in a car and did not put on a seat-belt you would get into an accident, would you start putting on your seat-belt? If you got into an accident your life would be changed FOREVER, due to possible paralysis, brain damage, or death. If putting on a seat-belt meant that you wouldn't crash, would you wear it? Even if you didn't like the way it felt or it made you uncomfortable, would you put it on? Remember, seat-belts save lives"
That's my opening line when I tell my kids about the birds and the bees.
I'm still coming up with my rebuttal for when they say, "what if I'm riding in the trunk?"