Most Helpful Guy
A guy who values you and worthy of you will want to take the time to get to know you a bit and ensure that both of you are ready to have sex, whenever that may be (and after the first date is generally not realistic, no matter how well it goes).
I'd recommend just telling him that you would want to know him better and have at least some more dates before considering having sex (and don't feel bad about saying you'd need to be in an exclusive relationship before you'd be ready for sex, if that is how you feel). If he can't understand that and is in too big of a hurry to actually get to know you a bit first, he's not worth your time.
Most Helpful Girl
Just say “I really like you but I need to take things slowly. I hope you can appreciate that”