Why do I keep doing this?

It might seem exciting, but if it's also distracting you from making relationships that can go somewhere then you should consider whether it is worth it. Presumably you're talking about things 'getting ugly' because your father would be upset by it. It's a familiar trope that some girls like to get into relationships their father would disapprove of, as a form of rebellion, so perhaps there is an element of that. You are a woman now, so you can make your own decisions about who you choose to be intimate with - just be wary of the fact that this older guy should know better too and perhaps he has been guiding you down a path that isn't really what you, as a woman, would choose on reflection. Saying 'Stop, before we get caught' is not really a mature and responsible way of bringing an end to something like this. I don't think the age thing is necessarily an issue, but the fact that he is a friend of your father makes it feel to me like he is abusing a position of responsibility.

On the other hand there's nothing wrong with it at all, as long as he's not married or seeing anyone else.You are both consenting adults and the fact that he is a friend of your fathers is irrelevant.

The thrill of forbidden fruit can be intoxicating. And you are correct that it could get very ugly...destroying the friendship between your father and this man. Damaging the trust between you and your father. The chance of getting caught can possibly be causing that much more excitement.
Need help ending it? Think of him as your dad's brother. You would never do this with your uncle. You can even start calling his name with "uncle" in front of it. That will remind him of the proper relationship as well.