Statistically, indeed, we are unlikely to marry a girl/boyfriend in teenage. However, we should try out best to make the relationship last as long as possible. Thus, we should keep thinking about marriage in the future. I believe that such attitude helps relationships to work better for both of them.
Just because you love or care for someone your seeing does not mean they share the same affection. Fixating on marriage would just lead you to heartache. Professing love or talking about marriage to your boy/girlfriend too soon will likely freak them out and make the rest of your relationship uneasy if it doesn't come to a crashing end. I'm not saying you should abandon the idea, Just try to keep some control over your own emotional attachment. If things don't work out for some reason, You won't be as heartbroken. Thing is, The longer it lasts, The more likely that both will fall in love. Till then, Enjoy the time you have with them.