"Inherently, I find that having sex and intimacy, when it's great, it's funny," the actor, 32, tells E! News.
That attitude was helpful on the set of his new movie No Strings Attached, in which he had a handful of sex scenes with Portman.
"[The scenes] are sort of meant to be funny or comedic," Kutcher says. "The real challenge was, you've got 50 people looking at you, you're with someone you don't really know, and you're trying to make sure they're comfortable, you're trying to be comfortable, and now, you've gotta be funny."
Kutcher also shares the secret of how he keeps his marriage to Moore, 48, on an even keel.
"[It's] constantly reminding myself, like, no matter how amazing my relationship is, keep working on it," he says. "Keep working on it and keep finding another level. Things can always get better, even when it's going great."