Prepon told Us Weekly she wasn't surprised the two are together and confirmed Kutcher was Kunis' first kiss.
Kunis famously won a part for the Fox comedy despite only being 14, telling producers "she would be 18," which was their requirement - but wouldn't say when her 18th birthday was.
During the show, it was scripted that the two kiss - but Kunis apparently didn't reveal it was her first kiss in her life. Kutcher told People of the revelation, "I'm so glad I didn't know that or it would've been too much pressure."
"I was so nervous. She acted so cool. She seemed as if she'd done it a thousand times. I was the one with the butterflies in my stomach. I mean, here's this little girl and I have to kiss her. It was nerve-racking."
Mila Kunis was photographed earlier this week with ring on her left hand, sparking rumors that she and Kutcher were engaged - although he has yet to finalize his divorce with estranged wife Demi Moore.
Prepon says that she thinks she would be invited to a possible wedding. "Obviously if people wanted us there, which she would because we're all family -- then of course I would be there. We support each other no matter what."