Just say, “hey I’ve had a really lovely time but I think our wants are a little different right now so I think it’s fairer to let you invest your time in someone looking to date/have a relationship.”

Don’t sleep with her. It’s her first ever kiss, her first ever date. She has no idea how to read signals yet. Don’t help create a ‘bad’ memory for her that’ll last many years. She may well alter her view and say she’s open to something casual but she’s been so vulnerable telling you of her inexperience that it would be a real shame if someone exploited that for an “easy lay”. I do admire you for thinking it through and asking for advice. I’d move on to another person. You’re not even that attracted. Yes another man will probably tell her what she wants to hear and exploit the opportunity but you shouldn’t want to be that guy. It’s not a nice feeling to have on your shoulders just for one night of probably mediocre sex (she’s inexperienced, you’re not that attracted).