Ian McKellen: What's wrong with us? Should we not aspire to happiness?

Ian McKellen at the Manchester Pride Parade in 2010 (Rex Features)

Any two people in the UK should be able to marry. The proposal is a modest one. Why then does it provoke such opposition from those like Lord Carey, who knows from his own experience "how wonderful marriage can be" and yet argues against gay citizens enjoying an equivalent marital bliss.

He thinks that same­gender marriages would "undermine the institution" and weaken it "fatally". Does that mean that Lady Carey and he would have been less happy together if I, married to my husband, had been living next door?

Marriage has been a beneficial institution of long standing, worldwide. It has, though, been open to regrettable interpretation – child brides, brides promising to obey, polygamy, arranged marriages, the ban on mixed-race unions. Laws were changed and what is good about marriage has survived. It continues to thrive in South Africa, where same-gender marriage is protected by the constitution. Would it not be just the same in the UK?

As an antidote to those heterosexuals who have brought marriage into disrepute and whom Lord Carey condemns, why doesn't he now welcome gay couples who want to accept its challenges? Not all gay people will choose to marry, of course. I bet Sir Elton and David, their son in hand, would be amongst the first. But why not celebrate those who want to marry and bring up a family? Why not change the law so more can do so?

Lord Carey grants us our civil partnerships but no more. What's wrong with us? Should we not aspire to domestic happiness? Why can't our relationships be recognised on a par with everyone else's? Lord Carey does not "begrudge rights and benefits to homosexual couples" yet he says that the idea of equality between gay and straight people is "the mantra of the equalities industry".

As one of the co-founders of Stone-wall I am familiar with such hints of homophobia. Stonewall has indeed pressured successive governments but Lord Carey has misunderstood our argument for equality, when he defines it as "being equal means being the same". On his more generous days, I'm sure Lord Carey would accept that all human beings are the same in being God's children.

A gay atheist shouldn't delve into theology. Same or not, all I want is to be treated equally under the law.

Lord Carey admits that marriage does not belong to the Church (indeed pre-dates it), yet it's as if somehow he owned marriage, that his definition was the only legally permissable one. But he surely goes way too far in opposing same-gender marriage because "it will encourage religious discrimination". He unconvincingly cites the fining of a Christian couple who would not shelter a gay couple in their bed-and-breakfast. "No queers welcome" has a nasty ring to it.

Dr Carey's trump card is a leftover from slavery, capital punishment, votes for women: "I do not believe the British public wants any of this." A Prime Minister on this issue should lead and not follow any supposed public opinion to the contrary. Anyway the British have accepted civil partnerships and may well support a humane and clear-sighted view of society, embracing anyone who wants to get married, for their own benefit and indeed for the nation's, regardless of race, belief or gender.