Whenever I start having sex with a girl, she wants to be my girlfriend. I get that this is what all the women will want, but how do you fight it off?

I screwed up recently. I'm banging this girl, and midway through, she asked if I want her to be my girlfriend. I said yes, as I wasn't in the best state of mind obviously. Now she's got a spare key to my flat. How fucked am I?

Should I just next her, or is there a way to get my key back, and put her back to a plate?