Should my 18 year old step daughter spend the night at our house with her boy friend.

My 18yo step daughter lived with her father, but now lives with her boy friend's parents, and in both cases she's able to sleep with her boy friend. Not too long ago we had a family gathering, of course she and her boy friend attended, but wanted to sleep together. I didn't think it was appropriate, and didn't allow it. I felt it would send the wrong message to the younger members of the family, and being that it's her first boy friend, the chances of them staying together are slim. I know she's an adult, but I feel values are important in my house. What are your feelings about this???