NEW YORK – Scarlett Johansson says that while monogamy might go against instinct, she’s happy in her relationship with boyfriend and recent “Black Dahlia” co-star Josh Hartnett.

“Josh is very sweet,” the 21-year-old actress says in an interview in Allure magazine’s November issue, on newsstands Oct. 24. “He’s a good boy. A great person. I’m very lucky and I’m very happy. Contrary to popular belief, I’m not promiscuous.

“There does seem to be a mistaken belief out there that I am sexually available somehow – which is not to say that I’m not open-minded about sex,” she continues. “Yet I wouldn’t say I’m a serial monogamist, either. I mean, I went through periods of time when I was, ah, single. But when I’m in a relationship, I’m in a relationship.”

But Johansson, whose films include “Lost in Translation” and “Match Point,” still acknowledges monogamy might not be natural.

“I do think on some basic level we are animals, and by instinct we kind of breed accordingly,” she says. “But as much as I believe that, I work really hard when I’m in a relationship to make it work in a monogamous way.”

Johansson has another confession.

“I get tested for HIV twice a year. … One has to be socially aware,” she says. “It’s part of being a decent human, to be tested for STDs. It’s just disgusting behavior when people don’t. It’s just so irresponsible.”