The actress says she's "very lucky" to be with "a great person" like her boyfriend

Finally speaking up about her relationship with boyfriend and Black Dahlia costar Josh Hartnett, Scarlett Johansson gives him and their relationship high marks.

“Josh is very sweet,” the 21-year-old actress tells Allure magazine in its November issue, the Associated Press reports. “He’s a good boy. A great person. I’m very lucky and I’m very happy.”

Where she does have some problems, the Lost in Translation and Match Point star suggests, is with her image.

“Contrary to popular belief, I’m not promiscuous,” she says. “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.

“Yet,” she goes on to say, “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.”

Then again, she opines, “I do think on some basic level we are animals, and by instinct we kind of breed accordingly. 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 also encourages responsibility when having sex. “I get tested for HIV twice a year,” she says. “One has to be socially aware. It’s part of being a decent human, to be tested for STDs. It’s just disgusting behavior when people don’t. It’s so irresponsible.”

Only last month Johansson, while noticeably mum on the subject of Hartnett, did admit to some other passions in her life, telling InStyle that she loves her shape.

“I’m curvy – I’m never going to be 5′ 11″ and 120 pounds,” she said. “But I feel lucky to have what I’ve got.”