In the final days of Question Month, she sends me one: "How do you define sex?"
And that's an easy one. For me, sex is pentrative intercourse.
As I've pointed out before here, I agree with our former President that blowjobs are not sex. It's foreplay, but not sex.
And I don't consider SM to be sex either. It can be sexual, and it can include sex, but if SM is sex, than boxing is sex, and extreme sports are sex, and riding a really good rollercoaster is sex. And they aren't. All those things involve adrenaline, endorphins, intimacy, and exhilaration. (When they're good, anyway.) And so can sex. But if we're doing an Thomistic defining of terms thing, then it's best to keep things simple.