I have a question.
How long have you been with this guy?
I think that if he's been begging you to move in, your talk with him is long overdue. You don't have to say "hey, I am sick of entertaining your no-life daughter" but since you ARE concerned for her (and you seem to be a very kind soul!), I would suggest you say something along the lines of:
"I've been thinking a lot about Katie lately and I have to say I'm really concerned about her. I love her, but I'm afraid that she is relying on us too much when most 18 year olds have jobs, cars, and social lives. I feel like it's definitely been putting some pressure on me, and it's changed the dynamic of our relationship. I was hoping that you and I could encourage her to do more with her life. This is a difficult situation for me to want to commit to as it is, which is why I haven't moved in yet. I just wanted to be honest with you so that we could make some changes together."
I hope he agrees with you and wants to change this life. But if he doesn't, you have some serious thinking to do about whether or not this relationship is right for you. Best of luck and please let us know how things progress.
I diagnose for a living so I've already thought of every disorder possible to explain away her behavior.
He is the biggest king of denial I have ever met.
I have no suggestions for making Katie get a job or go on a date, but I do have a suggestion for the interminable evenings of conversation: get some hobbies!
Thanks for the advice.