In today’s I’ll-never-look-at-them-the-same-way category, I always knew Mackenzie Phillips had an screwed up druggie childhood while appearing in “American Graffiti” and the sitcom “One Day at a Time.” But her latest admission is a whole new layer of freaky-deaky.

Phillips writes in a new book out today that she had sex with her father, musician John “Papa John” Phillips of the Mamas and the Papas, when she was 19. Ewww! It happened the night before her 1979 marriage to Jeff Sessler, a member of the Rolling Stones’ entourage. That marriage ended in 1981.

“On the eve of my wedding, my father showed up, determined to stop it,” she writes in the new book, “High on Arrival.”

“I had tons of pills, and Dad had tons of everything too. Eventually I passed out on Dad’s bed. My father was not a man with boundaries. He was full of love, and he was sick with drugs. I woke up that night from a blackout to find myself having sex with my own father.”

In 1980, she went to rehab with her father. By that point, her 10-year-long sexual relationship with him had become consensual. Ah, gross! She’s lucky she didn’t get pregnant and have a mutant baby with three heads.

This was going on while we were all watching her on “One Day at a Time” as the loveable but rebellious teen Julie Cooper Horvath — and you thought Schneider was disgusting! She was fired from the show and her character written out of the story at one point after relapsing from drug rehab. Ugh. I’d have to take lots of drugs too if I had to live with the image of having had sex with my mother. Ah man. Now I need to take about 10 showers after writing that last sentence, for real.

“Papa John” died in 2001. Phillips is now 49 and has appeared sparingly in TV dramas in recent years. As recently as a year ago, she was arrested at LAX for smuggling cocaine hidden in balloons and plastic bags. Hey dumbass, were you too stoned to realize that since 9/11, you can’t just go through airport security with a bunch of dope?

Phillips has two red-hot half sisters, actress Bijou (who posed naked in Playboy) and singer Chynna Phillips (the blonde from chick group Wilson Phillips).

He may have penned great songs like “California Dreamin’,” “Monday, Monday,” and “San Francisco (Be Sure to Wear Flowers In Your Hair),” but “Papa John” was a real, um, character. His years of addiction led to the need for a liver transplant in 1992. Several months later, he was photographed drinking alcohol in a bar. He told Howard Stern, “I was just trying to break in the new liver.”

I know it’s based on entrepreneur John Schnatter rather than the perv singer, but I bet the guys at Papa John’s Pizza are wishing they’d named the company something else today.