Diana, William and me

No, Elton does not bankroll his filmmaking. The much artier Women Talking Dirty, starring Helena Bonham Carter and Gina McKee, Furnish's second major venture (after the Bafta-nominated Tantrums and Tiaras, his documentary about Elton), lost him money, and he is determined to recoup it.

He wants to play Hollywood at its own game, and It's a Boy Girl Thing certainly holds its own against any big studio US equivalent. Its central conceit is the old body-swap routine, updated and set in the context of an American high school. Boy gets girl's body and vice versa.

It's a film, essentially, about the fluidity of sexuality, from one half of the world's most famous gay couple. Furnish, as he readily admits, has spent most of his life either fleeing or challenging sexual prejudice ("I ran away from small-town Canada, to London; I ran away from my family because I didn't think I could be the person I was") - though he now finds, happily, that there's less and less of it about. We talk briefly about The Archers, and the forthcoming gay wedding which is causing such controversy in Ambridge.

"I have to be careful when I talk about this, because certainly as a gay man I live a very rarefied life," says Furnish. "I don't live the same gay life as someone on The Archers, in middle England." He laughs at the thought. "But I have spoken to people who work on gay service helplines, and there is still prejudice out there. There's violence, problems with families.

"Elton and I were quite surprised that we only got one nasty card left at the Windsor Guildhall on the day of our civil partnership ceremony, though it was really nasty. Otherwise all the correspondence we had was nothing but positive."

It is almost exactly a year since Elton and David got "married", on the very first day of legal same sex unions in England and Wales. I'm vaguely shocked to hear that they were nervous about the public response. "We just weren't sure how people were going to react. We didn't know if we were going to get flour-bombed or spattered with eggs."

Did he really think people might throw food at them? "Yeah. We just weren't sure what would happen. In the end, we were blown away by the cross-section of people who came out to support us. It made me very proud to live in this country."

He thinks that the civil partnership legislation is as important, in terms of basic rights, as universal suffrage. "It's a seismic shift in civil rights. For me it's right up there with women getting the vote, blacks getting full equality and equal pay for people in the workplace."

This weekend he and Elton are going to the wedding of Little Britain comic Matt Lucas and his partner Kevin McGee. The theme is panto: Furnish will go as Prince Charming and Sir Elton as Captain Hook.

They have thought long and hard about children, the next big step, traditionally, after marriage - and have decided not to adopt. But it is an issue that clearly divides them. Elton once said that one of the great perks of being gay was not having to have kids, but Furnish has had a tough time resigning himself to childlessness.

"We both felt that if we had children, we wanted to be active parents and role models and we looked at our lives realistically and said it just wouldn't be possible." Elton, he adds, is on tour for 10 months of the year at least.

Yet Elton once wavered. They were in the roughest part of Soweto, South Africa, opening a childcare facility built by the Elton John Aids Foundation (which has raised $120 million, and funds the kind of grassroots projects, here and abroad, that rarely make headlines).

"It was one of those lightning bolt moments. This one little boy, a beautiful little boy who'd lost both his parents to Aids, came out to present us with a thankyou gift. He was supposed to go off the stage, but he didn't. He just stood there, nestled beside Elton's leg, then Elton picked him up and sat him on his lap.

"We got in the car afterwards and Elton just turned to me and said: I've never felt that calling before. And he said, wouldn't it be amazing to adopt a little boy like that.

"Then we looked into it, and it's extraordinarily complicated. We also found out that the little boy was living with his grandmother and his auntie and his brothers and sisters, and that was absolutely the best place for him to stay."

So how does he feel about the adoption of baby David from Malawi by Madonna and Guy Ritchie, both friends? "From what I understand, and I don't know the situation directly," he replies, "Madonna had done tons and tons of advance work. The papers gave the impression it was a much more immediate decision than it actually was. And I think she felt the media whipped it up into a frenzy. I certainly believe her heart is in it 100 per cent."

Later, Furnish says that his friends are, by default, his family. They often appear to befriend, I reply, the famous and fragile - the damaged actress or the confused sports star. "Well, Elton's very well qualified to offer advice to people in difficult situations from within the artistic and sporting world," he says.

Yet the gossip column stories aren't always true: they know Geri Halliwell, but "don't have her phone number". They haven't seen the Beckhams for ages ("I feel absolutely wracked with guilt that I have not got on a plane and gone down to Madrid and visited the boys in their own home. But there just has not been time. My priorities are Elton, my work and my commitments to the Foundation, and that's more than enough.").

Furnish suddenly leans forward, as if to tell a secret. "When Elton is away I'm really lonely, to be quite honest. I really miss him. When he's in a room you feel this amazing life force. And when that's gone, it's like a vacuum."

Furnish shows me to the door, pointing out a large canvas by a hot new New York artist as we walk. "I'm so excited about Christmas," he adds. "The fact that we'll have 10 days together with no professional or social obligations at all. We can just hang, watch DVDs, be together. Now that is luxury."