Consent.

He paused before entering me, his cock almost but not quite touching my pussy. "Do you want this?" he asked, breathily, and waited.

Well, that's a silly question; I was naked on the bed in front of him, face down ass up and spreading myself open for him, and thirty seconds previous I had been sucking his cock. And it could pass for just dirty talk, blending in rather seamlessly with less consequential questions about do I like his big cock and am I a naughty little slut.

But I liked it because it showed beautifully how easy, and how crucial, explicit enthusiastic consent is.

(I should note at this point, because I believe in shades of gray and all that, that not every sexual encounter lacking explicit enthusiastic consent is rape. Just that explicit enthusiastic consent is a really good thing to be sure about if you wish to be a better lover than "not a rapist.")

I could have said yes. I could have said no. I could have said "hang on, let's talk." I had power over what would happen to me.

And I said yes. I said oh yes and rolled my hips back onto his cock and we both gasped a little. I said yes and it was extra sexy because I had the chance to say no. I said yes and then he knew I wanted sex, wanted his cock, wanted him.

Consent isn't just an ugly little prerequisite to sexiness. Consent, breathed out in an "oh yes, oh please yes," is in itself sexy.