In the dying days of her leadership, Margaret Thatcher was fleetingly celebrated as a gay icon after being admired by the camp artists Gilbert and George. But her private views on homosexuality were assumed to be anything but liberal, particularly after she helped to power through a law that banned the promotion of homosexuality in schools.

Now, however, the Iron Lady could have the opportunity to set the record straight – by accepting an invitation to attend the gay “wedding” of her old friend Alan Duncan.

In a few months the dapper shadow enterprise secretary will become the first Tory MP to enter into a civil partnership when he ties the knot with his boyfriend, James Dunseath. (MPs who have already taken the plunge include Ben Bradshaw and David Borrow, both Labour.)

Apparently, at least 300 guests will be attending the ceremony, due to take place before the end of July. Duncan won’t say if Lady Thatcher’s invitation will be dropping through her letterbox soon, but it would be odd if she were left out. He and Thatcher have been close for years and often dine together.

There is no doubt, though, that section 28, the legislation that her government introduced to stop “loony lefties” teaching schoolchildren about gays, gave the Conservative party a long-standing and damaging reputation for homophobia. It is only now, under the leadership of David Cameron, that the party has finally shaken off that image, promoting instead an inclusive, nondiscriminatory attitude towards gays.

Duncan, for his part, claims that he hasn’t detected the slightest whiff of disapproval about his forthcoming civil partnership from even the most traditional figures in the party. “You should see my mailbag. I’ve had so much support,” he says. “People are saying, ‘Thank God for that – at last we can see off the last vestiges of sniping, sneering and disapproval.’ They are just writing to say, ‘Thank you, now the world can grow up and get real’ – and it’s great.”

But Duncan has never wanted to be branded as a gay crusader. Initially, when we meet, he insists that he wants to talk about heavyweight issues such as this week’s budget. It’s abundantly clear, though, that he’s head-over-heels in love – so it’s not hard to steer him away from politics to affairs of the heart.

Sitting in his living room, surrounded by snaps of the photogenic Dunseath, he brims with pride as he talks about him: “He runs the press office of the International Futures Exchange. You know, it’s massive. Bigger than the stock exchange.”

In fact, Dunseath is such a hunk, he teases, that both male and female friends have been saying that it’s a pity he’s no longer single. “All the women say about James, ‘It’s not fair, I’m really against this, it’s such a loss’. In fact, some say, ‘What a double waste’, which is really quite flattering.”

The pair met 14 months ago on a blind date and Duncan was instantly smitten: “Mutual friends set us up over dinner. We both said, ‘Don’t be ridiculous – we don’t do blind dates’. It took about half a second before we changed our minds.”

At 39, Dunseath is a decade younger – but the age gap doesn’t bother Duncan at all. “We have a very similar background – boarding schools at a young age, forces families, overseas postings,” he says.

“We’ve both got international experience which is quite a character-forming ingredient. I was brought up in Gibraltar, Nor-way and Italy; he was [brought up] in Kenya and Germany.”

As an MP Duncan could hold his reception in one of the grand function rooms in the Commons, but he doesn’t think this would be fair on Dunseath: “I want to get away from Westminster . . . I want it to be a shared thing.”

After the ceremony and honeymoon he expects that they will settle down at his town house – on the fashionable and appropriately named Gayfere Street, five minutes’ walk from the Commons. It was here that John Major set up his campaign headquarters for the party leadership in the hours after Thatcher announced her resignation in November 1990. Cameron, then a young party strategist working up to 20 hours a day, often used to doss down for the night on Duncan’s floor.

Back then the house was “chintzy”, Duncan says. Today it’s a contemporary bachelor pad: all top-of-the-range stereo systems, modern art and wall-mounted flatscreen televisions. There’s black textured wallpaper in the bathroom and a black glittery loo seat. In the drawing room are not just one but two large portraits of Duncan himself. “I should move one downstairs,” he says, with a flicker of embarrassment.

Among the guests at the civil partnership ceremony will be Duncan’s two brothers, his mother Anne and his nieces, who think his engagement is “just so cool”.

His father, an RAF wing commander, died in 1989 – more than a decade before Duncan publicly “came out” – and never knew that his son was gay.

“We never discussed it. He wouldn’t have minded. It was just never necessary to discuss it really,” says Duncan, although he believes now that families should be able to talk openly about such things.

“There are still teenagers who commit suicide because they get wound up about all this, and that can’t be good,” he says. “If it’s how you’re born, you should be able to talk about it without feeling humiliated. I’m all for a world where it’s so much a matter of fact, so inconsequential, that nobody need feel in turmoil about it.”

The role of schools, he believes, is as important as the role played by parents when it comes to helping young people who are struggling to come to terms with their homosexuality. And he is adamant that homophobic bullying needs to be stamped out.

However, he also thinks that, in some respects, the pendulum has swung too far the other way. Schools should be “confident enough” to adopt a more relaxed attitude to good-natured teasing of gay pupils, he says.

“Everybody’s being taught that if you tease someone nicely about their sexuality, then that is necessarily homophobic. Well, actually, we need to get through that barrier, to the point where you can tease because it isn’t an issue. I actually find that being able to laugh at myself and make jokes against myself is a very useful tool in breaking down barriers and making people feel comfortable all round. It would be a pity if undue correctness [put paid to] all that.”

Duncan was educated at boys-only schools: Beechwood Park prep school in Hertfordshire, followed by Merchant Taylors, where he was head boy. After going to Oxford he won a scholarship to Harvard, then worked as an oil trader before becoming MP for Rutland & Melton in 1992. But it was another decade before he “came out”.

This time last year he would have laughed if anyone had suggested that he would enter into a civil partnership: “I never expected it. Didn’t think anyone would put up with me. It’s a busy life in public life. You think people would say, ‘I don’t want to take that on’. But it clicked and it’s great.”

I ask if he thinks Gordon Brown is committed to promoting gay rights. “He certainly gives the impression of being very uncomfortable about issues of this sort,” replies Duncan. “It’s the sort of thing that seems to send him into hiding. Whereas Tony Blair was totally confident – really a fantastically constructive influence on this [issue].”

Duncan counts steering the civil partnership bill through the Commons as his proudest achievement, although he would be frustrated to be remembered only for his sexuality: “I’ve always said I’m a politician first and gay second. From now on I hope [our sexual orientation] is just a matter of fact for me and for others and we can get on with being good politicians.”

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