What if she start talking about sex

A question that I often get is, "What if a woman starts
talking or joking about sex on a first date?"

In my experience, this is often a test.

If you become uncomfortable with her sexual comments,
she's going to know that you're not uncomfortable with your
own sexuality.

In other words, you fail the test.

Or, if you get all excited and talk about sex with her,
you also fail because it shows you're quite desperate. (A man
who gets plenty of regular sex has no reason to get enthusiastic
when the subject comes up.)

So...how do you respond?

Just smile coolly as you look at her in the eyes, as if
you've "got the joke" but don't think it's a big deal. Just
casually react to her, as a man who gets plenty of sex already
would react.

Or...to take things one step further, you could look
it her in the eyes and then in the lips to create sexual tension,
as if you're thinking, "I know what you're thinking..."