I'm leading my girlfriend on because I really don't want to get married.
Why don't you want to marry your girlfriend?
I'm still young and not ready to settle down yet.
It's fine to not want to marry your girlfriend because you're too young. But it's wrong to lead someone on when you never plan to propose. You should be honest with her.
I can do whatever I want to do, I don't owe her anything.
You really should tell her how you feel, she may not be ready for marriage yet either. If she wants to get married, she should know that you're not ready for that, you do owe her that much.