Awkward situation, What would happen if a single dad of a 16 year old boy and a single mom of a 16 year old girl hook up and get married after their kids have been boyfriend and girlfriend for years? They have been HS sweethearts can they still marry?

Well first of all, it is legal. Second, they can choose whether or not to continue with their relationsihp. I really don't see anything wrong with that, but it is just rather uncommon.

I think this was a TV sitcom!
Yes, it's legal, nothing wrong with it, since they are not biologically related.
The only problem I can see is if they break up later, yet still have to be stepbrother and stepsister.
Even then, by 16 they should be able to adjust and deal with it.
Lot's of people have to continue and see their X's every day. :)

Of course they can. No blood relation. Be happy first!

This happened to one of my friends actually. As it happened she felt too strange about continuing the relationship when her dad was dating her boyfriend's mum and they finished it - which I think is probably what happens in the majority of cases. The situation is strange, and potentially could cause problems (what happens if the parents split up in a bad way - will it affect the youngsters' relationship?) but they're not blood relatives- legally they can marry.

Yes they can still marry, as many people here have said already. It does remind me of a place in North Carolina mountains I visited a few years ago though... apparently if a man and wife get a divorce there they are still brother and sister.

Nothing bad would happen, but it would be weird for the children to be dating one another. It is legal.