How can start feeling positive about relationship a son or daughter gets into that you don't 100% approve of?

My stepson, i consider him my son is in the navy and hooked up with a girl who is 18 and met on a chat line that you pay to show her semi naked. They have been seeing each other for month . She came down to to visit him ,and NOW is moving in with him and his roommates . Today ( he is on our cell phone plan with Sprint ) he added her and his father APPROVED. My husband has a soft heart for people in need.I am a softie but not as much. I pay our bills and I worried and don’t approve. How can I change my mind?I am not heartless. This girl does not like her living arrangements at home.I am at a loss. My stepson is 24. I worry about him with all this. I am worried about him more. I can not tell him what to do. Thanks Fluther I feel better. I ranted . Thanks for anyone who listened.