He is the creator of galaxies, saviour of Saturday night telly and the most influential gay man in Britain, but Russell T Davies can still shriek like a starstruck fanboy. "Richard Dawkins!"

The evolutionary biologist and best-selling author of The God Delusion will appear as a guest star in the new series of Doctor Who, which began last night. "People were falling at his feet," says Davies, creator of the BBC's flagship show. "We've had Kylie Minogue on that set, but it was Dawkins people were worshipping."

As writer and executive producer of Doctor Who, Davies often plays with religious imagery (from a cross-shaped space station to robot angels with halos), but he's a fervent believer in Dawkins. "He has brought atheism proudly out of the closet!"

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Russell T Davies is a big man squeezed into a smallish leather armchair, at a private members' club in London. Six feet six inches tall, he waves his big hands around a lot... except when he's pushing the chunky, black-framed glasses back on his broad face, or running a hand through fluffy hair that, frankly, won't take it for very much longer.

Davies was born not too far from the birthplace of Tommy Cooper in South Wales, and seems to have borrowed his chuckle from that melancholy-jolly comic. When we meet he is psyching himself up for the star-studded series premiere, but, despite his physical and professional stature, his enthusiasm for Dawkins makes him go as giggly as the fans who thrust posters of the Tardis at him to sign. (If you are one of them, speed-reading for clues about what will happen in the show, stay with us. Dawkins is an exclusive, and there's a cracker to come.)

"He was as mad and as barking as you'd want him to be," says Davies, putting on a magisterial voice to imitate Dawkins: "'Don't touch me, I've got a cold! Don't shake my hand!' Just brilliant. It's like, 'Woah!'" He didn't have the nerve to ask the professor to slip on a latex mask, it seems: Dawkins will appear as himself. "His wife, [the actress] Lalla Ward, used to be a companion of the Doctor. The gays loved her."

Ah, "the gays". Last year, an Independent on Sunday panel put Davies top of our Pink List of the 100 most influential gay men and lesbian women in Britain. "Isn't it just nonsense?" he says, laughing, although he was clearly flattered. "Did you see the photo? I was like a prancing clown! I think the influential ones are the ones fighting for the laws and so on," he says, then concedes: "I'm not without influence. I'm very out and proud and deliberately visible with it."

Why place him higher than Stephen Fry, Sir Elton John or Peter Mandelson? Partly because of the status he has within his industry, achieved by doing the impossible: reviving the Doctor – turning a dusty old joke into a witty, sexy, slick and scary show – and making Saturday tea-time family telly compulsory again. But also because of what his critics call "the Gay Agenda".

Davies caused a kerfuffle during the first series, back in 2005, by having the Doctor flirt with a new, openly bisexual character called Captain Jack. There was a lot more fuss when they kissed on the lips. Since then, Jack has spun off into his own, very adult series, Torchwood, but he is returning (along with former companions Sarah Jane, Martha and Rose, her old boyfriend Mickey and her mum Jackie). Significantly, the show has evolved. It now encompasses all kinds of sexuality without fuss, even from the tabloids.

"I keep thinking, 'Where are the headlines about this in The Sun?'," Davies admits. "There has been a cultural shift." He loves the thought that somewhere a young boy might watch Captain Jack with his family and say, "Actually, I've got something to tell you." "That's it! That kid is still rare, but it has started. If there is one kid now doing that, then in 10 years' time there will be thousands of kids, and 10 years after that, every kid who wants to will be doing that. Isn't that brilliant?"

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The playground has yet to catch up with its heroes: boys and girls are still being bullied for their sexuality. But, as a 44-year-old man, Davies is "massively jealous" of the increasing numbers of teenagers able to be openly gay. "There are still thousands closeted, but they are a proper little subset of gay life: 'out' 15-year-olds. It's the most magnificent shift in the whole culture."

Born in 1963, Davies did not come out to his parents until he was at Oxford University. What if he'd done it 10 years earlier? "My parents would have been fine. They wouldn't have chucked me out. There would have been silence," he says. "Nobody had the language then, in Swansea in the Seventies. Not even my parents, who were both Latin teachers. It's one of the great gay myths, the chucking out of the teenager."

After a degree in English literature, he became a BBC trainee (inventing the "T" to differentiate himself from a Radio 2 DJ). Somewhere in the vaults is the one episode of Play School he presented, but Davies quickly moved on to writing and producing. And enjoying the wild life brought to screen in Queer As Folk, the explicit but beautifully written 1999 series about young gay men. It broke all kinds of taboos, and caused bemusement back home.

"My mum said, 'This is porn!' I said, 'No it isn't, it's life!'" She usually narrated television for his father, who had gone blind. So how did she describe the masturbation scenes? "I don't know," he says, laughing. "I never had the nerve to ask."

It seems a long time ago now. "The fury of that sex drive, the madness of Canal Street [in Manchester], I wonder how much of that comes from repression when you're 12 to 20?" he says. "I'm not saying it's going to go away, because it's fun, and we had some good times, but what is thought of as 'the Scene' is an explosion that happens once you've left home, have your own wage, and can become whatever you want."

His own life calmed down seven years ago, when he met a customs officer called Andrew in a nightclub. They live separately in Manchester (although Davies also has a flat in Cardiff, where Doctor Who is made) and see each other only at weekends. "It's not that a portcullis comes down if he pops in on a Wednesday for a cup of tea, but, actually, he's got the patience of a saint. I didn't see him this weekend, because I had to rewrite the Christmas special." Davies stops himself. "He hates being talked about in interviews."

They were together on the day the show returned, in 2005. "There was such huge anticipation about it. I remember getting into the car with Andrew and he was like, 'I bet if you turned on the radio now they would be talking about Doctor Who.' He switched it on and they were! Jesus Christ, it was white hot." How did that feel? "Lovely! Fuck, I'm not going to be modest about that. It was lovely."

The Doctor makes millions for the BBC, so perhaps it should listen to Davies's anger about the time it is now being shown – 40 minutes earlier than before, at 6.20pm. "It's a shame. It's a terrible slot. We will lose viewers. I am unhappy. We'll see." Expect to see it moved back, later in the run.

Other sci-fi writers make a point of praising their hardcore fans, but Davies can be very rude about them (look away now, Whovians). "It's like having a swarm of fucking mosquitoes buzzing around you. It doesn't stop you doing your job, but, Christ, they buzz!"

They are buzzing furiously about the casting of the comedian Catherine Tate as the gobby new companion, Donna. At 39, she is much older than her predecessors, so isn't it hard to see little girls pretending to be her, as they did Rose? "I completely disagree," says Davies. "When I was an eight-year-old fan I could not have told you how old Sarah Jane was. Also, there's a heartiness, a boldness and a childlike glee to everything Catherine does. Kids love her. Her catchphrases are in every playground."

Tate still gets to be funny. This coming Saturday she asks a young girl in Pompeii where she hangs out with her mates: "D'you go to TK Maxximus?" But she also has to cope with the trademark emotional lurch in every episode, designed to tug heartstrings even amid sci-fi madness.

The BBC promises this will be even more intense in episode six, "The Doctor's Daughter". Is Davies going to say how come the most eligible – but elusive – bachelor in the universe has got a child? "No! Shut up! Mind you, we'll never get to week six without someone blabbing." Out on the fansites, they're guessing aliens will pinch the Doctor's DNA. But that sounds too prosaic, and not enough to justify the hyperbole of a writer who saw part of the episode and calls it "one of the single most audacious moments in Doctor Who's 45-year-history... cheeky, hilarious and brave".

The Child Support Agency is involved, says Davies. Donna is "literally like Jiminy Cricket on his shoulder, telling the Doctor what to do as a father. She says, 'I've seen men like that round our estate, with pushchairs and prams. You've gone into dadshock.'"

So here's your pay-off, Whovians: the Doctor is going to have a baby.

It's just a theory, but if a post-operative transsexual from Oregon can become pregnant in real life – as reported last week – then a fictional 900-year-old from Gallifrey with two hearts certainly can. There. That should get the mosquitoes buzzing.