One thing you can't do is back track. You can't now expect him to accept that you aren't going to sleep with him anymore. You can't tell him it was an accident and you slipped and fell on his penis. You did if, now the first thing you have to do is own it. The way you handle yourself from here forward is critical. You act as if it's no big deal, after all, you are a grown woman. So what if you slept with him too soon. It's no big deal. Many long term relationships develop in spite of this. Sex on the first date or first few dates is more common than you can imagine.
Whatever you do, don't apologize for having slept with him too soon. This shows weakness and insecurity on your part. Men actually have a name for this. It's called Anti-Slut-Defense, also known as ASD. You don't want him to think you have no control over yourself and you don't want him to know he affects you so much. It was probably pleasurable for both parties, so no need to apologize. A guy wants to be stroked after sex, not shot down with your crying and emotions.
You don't want to call attention to the fact that you slept with him too soon. You don't want to have a discussion with him about it. Chances are if you give him the song and dance about how you just didn't mean to have sex with him on the first date or so he won't believe it anyway, so skip it. If you try to talk about it, you give him the impression that you are over thinking and he will see it as you already wanting to be his girlfriend. What ever you do, don't start acting like his girl friend now. Most all girls do this and there is a good possibility he is expecting this from you. Just because you had sex with him does not make you his girlfriend.
Now there is a chance you can become just a booty call to him if you aren't careful. There are ways to avoid this though. I call it tap dancing around. If he starts calling late at night, don't answer the phone. If he only suggest hook ups, don't go. If he wants to just hang out at his place or yours, avoid it. In other words, if he isn't willing to go out and do things that don't involve sex, just don't go. This tells him with your actions and words won't be needed. Actions are way more effective on guys than words, trust me on this.
Next date, have your escape route planned for the end of the date. A reason you have to get up early etc... Give him a passionate kiss at the end and tell him you hate it, but you gotta go. Leave him guessing and wanting more. If you had sex on the first date or too soon, don't fret, you can still get the guy.