No, I'm not being old-fashioned

Zak, 16, wants Chloe to sleep over in his room, saying no one can stop them having sex. Isobel Lewis finds a practical solution

"MUM, IS it all right if Chloe stays the night?" asks my son, Zak. He's 16 and, as he often reminds me, an adult. He means spend the night, not on the sofa, but in his room, surrounded by dubious posters and discarded boxer shorts. "Umm," I say, "What does Chloe's mum think?" My son gives me a look. "If you're talking about sex," he says, "You're not going to stop us by saying she can't sleep in my room. Be reasonable, mum."

While Zak and I argue, his two younger sisters gallop in and out of the kitchen, saying things like: "Can I watch Neighbours?" and "Don't worry, it'll be heroin next." My son throws in his trump card, so valuable to "enlightened" parents: "You'd rather know about it, wouldn't you?"

My son has known Chloe for ten days. A waif-like figure in layers of diaphanous fabrics, she's moved from the other end of the country. She has an air of defiant vulnerability and stripes of pink in her long bleached hair. "She's the kind of girl you want to look after," Zak says. We argue amiably.

"I'm young," he says, "I want to enjoy myself." "This is a serious matter," I say. "Sex is fun," Zak counters, "It's a wonderful thing, you said so yourself. Imagine you were 16." I can't, just then, imagine being 16. My mind has latched on to a terrifying thought: "She might get pregnant," I say. "I'm not stupid, Mum," says Zak, "Why are you being like this? You're becoming a fuddy-duddy."

Boys of 16 like Zak are not worrying about essays, or forging glittering careers. They are thinking, fairly obsessively, about girls and sex. They buy horrible, lurid magazines with busty blondes on the cover. Parents, they believe, agonise too much. Smoking dope, sex, whatever: it's no big deal.

Zak is no problem teenager: he's never skipped school, his friends are great, he even hands in his A-level course work on time. He's thoughtful and, mostly, responsible. People like him. So what's all the fuss about? There is, still, a perception that teenage sex isn't something parents of boys need worry about: boys don't get pregnant. Zak's stepfather, falls into this category: "Good luck to him," my husband says.

But I don't feel like that. Zak is living in our house and will be for some time. Just because he's 16 doesn't mean we have no further influence on his actions. If anything happens, it will be me picking up the pieces. It'll be me doing the explaining.

I phone the mother of one of Zak's friends, who, I'm told, "has sex loads". His girlfriend also lives in an outlying village and often stays over. What does his mother do? "We have a rule that girlfriends spend the night downstairs on the settee," she says, "You can't stop them having sex, but you can enforce the sleeping arrangements. I ask myself: `What if my mother came to stay?' "

Another friend, a teacher, thinks differently. "When it happens, I'll move my son into the loft, as far away from us as possible," she says, "And I'll get him a double bed. Then at least they'll get a decent night's sleep. What I can't bear is the thought of kids creeping about. It would make me feel anxious and guilty. Much better to have it out in the open."

In the end, I follow my instincts and do a deeply uncool thing: I phone Chloe's mum. She's almost rapturous with gratitude. "You don't know anything about Zak's family, you've only just met him," she's been telling her daughter. My son's assured me that Chloe's mum is OK about her daughter staying the night, but when I ask for her views on the room-sharing bit, she's shocked. "Oh my God," she says, "We haven't come across this before."

I suggest tentatively it would be a disaster if her daughter got pregnant. She immediately points out that my son would be paying maintenance until kingdom come. It's a chilling thought. "I can't stop her having sex," she says, "But when they stay at our house, Zak will sleep downstairs."

Sex education at my son's comprehensive has been very thorough. He can quote you the failure rates of every method of contraception. Within days of meeting, Zak and Chloe go to the family-planning clinic. They make an appointment for Chloe to have a long-acting injection of progesterone and come away with a bag of free condoms. All this I discover much later.

Later I realise my son has moulded me into the kind of mother he wants, laid-back and liberal. He's stolen a march on me by putting me on the spot. In the end, it's all about practicalities: can Chloe stay the night? When I reply that she must sleep on the sofa, my son says: "Really, Mum, that's most inconvenient."

Three months have gone by and we've arrived at a compromise. Chloe is, I think, still spending a token amount of time on the sofa. My daughters are blissfully unaware of what's going on in my son's bedroom. "Do you think Zak and Chloe have snogged?" our nine-year-old asked recently.

The situation is not what I'd wish for. Life isn't like that. And my son is right: it is better to know.