You want to avoid suggesting that you’re more into her than she is into you. So “we’ve been on one date and now you’re my girlfriend, right?” is usually a bad idea. But “we’ve been on one date and I’d like to go on future dates” is probably okay (if she doesn’t want more dates, it wasn’t going anywhere anyway).
(Massive overgeneralization, of course, and also I’m not qualified to talk about this.)
Not if she also has a preference for you.
You want to avoid suggesting that you’re more into her than she is into you. So “we’ve been on one date and now you’re my girlfriend, right?” is usually a bad idea. But “we’ve been on one date and I’d like to go on future dates” is probably okay (if she doesn’t want more dates, it wasn’t going anywhere anyway).
(Massive overgeneralization, of course, and also I’m not qualified to talk about this.)