How do I explain this to my daughter?

My daughter is 18 yrs old, when she was 16 she met Ryan and they dated for 6 months and she got pregnant. Ryan was very supportive during the pregnancy but once my granddaughter was born he was immature, spending a lot of time with his friends and they broke up. He still comes around to visit and such. About a month ago I over heard her telling a friend that she "hooked up" with Ryan at a party. When I asked her what she meant she said they just hung out. Yesterday when I came home from work they were alone in the bedroom, when they came out they looked like they were doing more then just hanging out. When I tried to talk to her about it she said "were friends and co parents mom., " I don't want to be the over bearing mother but I can't have her having with a guy that can't commit to her, How do I get her to see that a relationship like this will only leave her hurt.