WOAH. I can think of 100 things that could go wrong if a guy started dating me with the expectation of marriage. The first being the guy expects we might get married. That is a lot of pressure to deal with along with some strong feelings that I probably won't share in the beginning of a relationship. To decide someone is the one (or might be) based on an agreement to date is nothing short of crazy in my book.
Dating "for fun" isn't necessarily a good reason. I would hope you begin dating someone because you find their company more enjoyable than an average friend and there is some mutual attraction there.
Dating for me has always been a learning experience. Everyone has an idea of what they want in a partner, when you date someone that image will change, regardless of how sure you were. You begin to discover what you really want and like- not what you thought you did. You will also see qualities you value and be able to pick those out in certain people. (Maybe even people you hadn't previously considered "your type".)
Dating is truly trial and error. Never shun experience.