The Oz: The Great and Powerful star took part in a smouldering cover shoot for Allure magazine, and was asked about her relationship with Ashton, who she has known for 14 years after working together on That '70s Show, and is thought to have started dating last year.
Mila said: "People change. So who you knew years ago is not necessarily the person that you [know now]... And who somebody was yesterday is very different from who they will be tomorrow."
The pair have been pictured spending time together around the world in recent months and were snapped kissing in Central Park in September.
But Mila said: "You can go either way. You can be in a relationship. Or you could be having fun."
And pushed further on her relationship status she said: "I didn't think this was going to be about my friends. Happily about myself! Not about who my friends are, who I'm dating, or who I may have ever dated. That's not talking about myself!"
But the 28-year-old star - who split from Macaulay Culkin in 2011 after nine years together - implied she was no longer single.
Mila showed off her curves posing in a plunging, see-through chainmail dress, but revealed she has a strict nudity policy when it comes to films.
Talking about her sex scenes with Justin Timberlake in Friends With Benefits she said: "I showed side boob. I don't need to show a**. You get one or the other. You don't need both."