I used to think there was nothing worse than imagining your own parents having sex. I was wrong. You know what's worse? Learning your parents' sex life is more interesting than your own.As a kid, sex was something I never really discussed with my parents. WASPs generally don't talk about such things. My mom never talked about sex with her mom, and she never brought it up with me. My dad died when I was 10, and though I knew my mom dated guys, I never thought about what went on after I toddled off to bed.Chaste and pure, not pawing and petting: That's the way we like to think of our folks. During childhood it's easy to keep that illusion. Until we're teens we never consider what goes on behind mom's bedroom door, and once we do, we try never to think of it again.But now my mom is 81, and all of a sudden she's started talking about sex. I know, I know -- I should be mature, supportive of her sexual identity, and I am, intellectually, but there are some things I'd prefer to stay ignorant about. No matter how much my cerebrum says "Okay," my gut still sort of shudders at the thought of her, you know, touching the monkey. The really weird thing is, a few months ago my mom's sex life became an open book. Literally.She decided to write a memoir discussing the men in her life. It turns out there have been rather a lot of them -- romances and hookups, big names and big drama. She asked me to proofread an early draft, and if you think talking with your mom about sex is awkward, try reading about her romances, page after page, paragraph after paragraph.The book is titled "It Seemed Important at the Time," and it's really well written -- sexy, funny, and smart. If it had been written by anyone else, I wouldn't have blinked at the content. But it's not anyone else; it's my mom, and reading her description of her current boyfriend as the "Nijinsky of cunnilingus" was kind of shocking. It's not really a visual image I wanted to have. The truth is, I don't know much about dance history, but I'm guessing Nijinsky was creative, or at least very limber.My mom is Gloria Vanderbilt, and she's been in the public eye since she was born. She's always been extraordinarily beautiful, and even as a kid, I knew men found her irresistible, but I was always happily hazy on the details.When I was about 8, I remember looking at a Richard Avedon book about beauty, and there was a striking photo of a young woman staring seductively into the camera. It was my mother, though to me the woman had no relation to the person I knew. That wasn't my mom.I guess I always knew she had a history, as they used to say -- after all, she had been married four times. I remember when I was a kid, we'd be watching an old movie and I'd ask her if she knew one of the actors in it. "Oh, yes," she'd say wistfully. She never went into specifics, but even then I knew that yes was packed with meaning.In school, whenever I read a 20th-century-history textbook, I kind of assumed my mom had at least met many of the main characters: Marlon Brando (check), Frank Sinatra (check), Howard Hughes (check). I just never really thought about how she knew them.I now know Howard Hughes used to take my mom for night flights above L.A., just like in "The Aviator. " (Now, thanks to Scorsese, I can't stop imagining my mom with Leonardo DiCaprio.) She also hooked up with Sinatra while she was still married to her second husband, a famous conductor; and as she lay in bed with a young Brando, she noticed he kept a framed 10-by-12 photo of himself nearby.By the time she was 18, she'd had romances with some of the most well-known people in the world. When I was 18, I was still watching late-night public-access TV and popping zits.My mom has never been a typical mother. She's very cool, and way ahead of her time. On report day at school, she'd show up dressed in a purple beaver-skin coat and matching stockings. Where she found a purple beaver I have no idea.She's not the milk-and-cookies type. Growing up, the only snack food we had in the house was Carr's water biscuits. You know, the dry crackers people use for cheese? Yum. I always knew she was different, but until I read her romance memoir, I never really saw her as a sexual being.I was in a bookstore soon after the memoir was published, and two teenage boys were looking at the cover photograph taken of my mom in her early twenties. "She's hot," one of them said. "Yeah, totally," the other responded. I nearly slapped the book out of their sweaty little hands.Why does the thought of our parents having sex bother us so much, even as adults? I suppose a Freudian would say we never get over the Oedipal idea that our mothers shouldn't have feelings for anyone else. Or perhaps it's because in our age-obsessed culture, sex is always viewed as a youthful act.If our folks are doing it, they're stealing the one thing we have over them. I think the reality is much simpler: We don't want to think about our parents as real people, with needs and desires, fetishes and faults. But that, of course, is exactly what they are.The book got great reviews and a wonderful response. It's taken me a while to adjust, but I think I've finally gotten used to the notion of my mom as a hottie.When I suggested she take the whole cunnilingus thing out of the book, she just laughed and told me I should have a sense of humor about it. She's right, of course, and that's the most embarrassing thing of all. I'm 37 and my mom is still able to teach me something about sex.