There is nothing that can prevent you from marrying your girlfriend unless you broke up. If you have been given the option of marrying your girlfriend, then you should be glad enough as this is someone you have known with both the weaknesses and the strengths. However, this differs from one person to the other as people have different assumptions.
You don't like her anymore. She has bad hygiene and you can't stand to be near her with a 50 ft pole. You're not ready for marriage. You think she's insane and will try to harm you in some way. You'd rather have fun unmarried for a few years instead of making a lifelong promise.
I don't think my boyfriend wants to marry me. I'm not even sure I I want to get married, I feel that I do. But I don't know if my boyfriend makes that cut for me, I mean I can want to get married but unlike other girls its not the marriage that I want, I want the right guy. A guy that really gets me and is there for me, as of right now I can be honest and say I have no clue if its my boyfriend yet, but I want to get married and be enough and be happy.