Dave, I wasn't trying to be a dick. I'm sorry if I came off like that.
If this girl was independent, living on her own, etc then I wouldn't see a big problem with it. But she is 17 and living at home with mom and dad. The whole "parents are away for the night, what do you like for breakfast" thing makes me believe she has this house fantasy in her mind. I would be a little leary of that. I think she is getting more attached than you think she is. For your own safety, and perhaps this girls mental health, I would call it a day.
There's plenty of hot girls in the 20-30 range. And if one of those gets their heart broken, you don't have to worry about her crying to mommy and daddy about it. Because if that happens, you'll have a pissed of father, or worse at your door step.