I'm going through something similar right now, but the girl doesn't have my number. I'm starting to think she wants more, though. We were supposed to be "occasional f*ck buddies", but now I think she wants more.
If the time comes, however, I'll have no problems telling her that it's over. We discussed things beforehand, and I made no promises. I won't feel guilty or mean in letting her go, if she starts to get serious.
This whole thing is kind of funny to me. It's the second or third time that I've had a ONS decide that she wanted more. I always thought it was guys who faced this problem since we're the alley cats who like to sleep around. I never thought a guy would want to get serious after a ONS. This reaks of AFC. It's a definite case of him not understanding, and having oneitis. He should recognize it as a ONS and move on. Just tell him that the sex was great and just what you needed at the time, but that's all you wanted.
So what if he gets upset. So what if he thinks you're slutty. It's okay for women to embrace their sexuality. I don't care what society says. If more women thought that ONSs were fine, guys like me would be a lot happier. Sometimes, sex is all you want. Sometimes you have to be mature enough to know that you're not ready for anything more serious or committed. Sex shouldn't be a committment for a relationship.
This guy needs to understand that what you had was a fun fling, and nothing more. He needs to be a man about it, and get over it. You didn't go on a real date. You didn't make any promises. You don't owe him anything, and that includes being nice. Give him the straight talk. If he can't deal with it, refer him to this site and maybe he'll grow up.