For now, it is important to put this relationship in perspective. This man is your boyfriend, not your daughter's boyfriend, friend, father, or stepfather. Likewise, it should not be his role to discipline her. I suggest that you create a clear separation between your girlfriend/boyfriend relationship and your mother/daughter relationship, and skip the "family" threesome. This means only seeing him in settings that do not involve your daughter (i.e., not at your place when she is home).
Your daughter is nearly grown up and will probably move out on her own in a few years. I don't see a compelling reason to move your relationship with your boyfriend into anything more serious or committed at this point. It's reasonable for you to remain involved, but I suggest postponing any long-term decisions until after your daughter has left your home.
Susan King is a wife and entrepreneur in Minneapolis, Minn.