Hi Nick, no your behaviour is totally normal for a guy your age. Say if you were to stay with any one girl for a long time, then get married, and the marraige then to not work out, people may say, you married to young, you hadn't dated enough girls to know what compatible meant!
My dad got married at 19 and his wife 17, five years later they divorced. When he later remarried and had me, I remember when I was growing up my dad telling me to date as many guys as possible, to get to know who was right for me. (not suggesting I was to sleep with them ofcourse, just to date).
I would say that once you have matured enough, and got through this stage, you will naturally find the right girl, and want to settle down.
The phrase settle down, is just that, at the moment you are not ready to settle with one.
Have fun whilst your still young.