Ask them what they would think if you got a girlfriend. Remind them how mature you are and that you wont threat her wrong , you are serious about being with her. Talk about her in a really good and high respect way.
You'll have to come forward sometime. If they dislike it than tell them they are going to have to live with it because it's who you're happy with not who they want you to be with. They should understand if you sit down with them and talk to them about it.