Okay, it seems to me that all the responses here are of the "Yeah, we say no sex on first date, but we might do it if everybody is okay with that." I was looking for a response from someone who is adamant that they wouldn't, no matter what. Thanks for your input though!
As far as not knowing what the other one is thinking, my GF and I have clues we can say without anyone knowing what we're talking about.
Case 1: One of us likes the situation, and wants to know if the other does.
Me: You know, I really liked that bottle of Merlot the other night.
(In fact, neither of us likes Merlot. So the mention of it alerts her that I'm interested.)
Her (If she agrees that it's on.): Yeah, that was great, we should get another bottle.
Her (If she doesn't want to take it further that night): I don't know, I thought it was a little bland.
That is, agreement about Merlot is agreement, disagreement is let's-think-about-it.
And, of course, she could start the conversation as well as I could.
Case 2: Absolutely nothing is going to happen.
Either of us: Hey, we haven't been to the zoo in quite some time.
Neither of us is wild about the zoo, any mention of it is a wave-off.
You can pick your subjects based upon your own lives and desires. It's just got to be something that wouldn't normally come up in conversation.