I have a teen girl, and she has a few guy friends that are dad's (they are older than her and they are just friends) and if she was the same age I wouldn't necessarily discourage it if the general character of the boy was a good one.
The more you discourage boys or girls on relationships or anything else they want to do, they more you will push them into doing it. In my opinion leave them alone and hopefully they will realize it's wrong before it's too late. If not, we all have to learn, and sometimes the hard way. I'm not talking about little kids or young teenagers.. I'm talking about older teenagers.