One of my 25 year old twins and I just had a talk about this yesterday. She is dating a young man I like very much. I said that, to move in together before marriage is a very bad idea. She agrees. Girls like mine are out there :)
These are the values that our family has, and I don't want the children to see me aprove of anything different.
I wouldn't do it either. The other twin has been dating a great young man for 4 years while they finished school (she just took her bar exam). They have their own places and, while I am indulgent with my girls, they know I am not indulgent about this. Hang tough.