President Obama has moved with the flow of history, at last. He now joins Prime Minister, David Cameron, and the new French President, Francois Hollande, in backing same-sex marriage. It’s one of the great moral and human rights crusades of our age – and it’s an unstoppable global trend. There is a growing momentum to end the ban on gay marriage in more and more countries, from Cuba to Finland, Uruguay, Nepal, Denmark, Brazil, Australia and Colombia.

Obama’s support will have a huge positive impact. It will boost the worldwide campaign for marriage equality and, through media reporting of his support, raise awareness of gay marriage among billions of people in every corner of the globe. Even people living under tyrannical, homophobic regimes will hear the message of gay equality. Bravo!

It is significant that Obama’s change of heart was influenced by the sacrifices of US lesbian and gay military personnel and by discussions with his family and friends, including gay White House staffers and gay parents at his daughters’ school. This concurs with existing evidence that people who know a gay person are more likely to support gay equal rights.

Endorsing marriage equality was a brave move by Obama, given the upcoming presidential elections and the fierce, powerful opposition he will now face from republicans, shock jock radio hosts and religious conservatives. Ending the ban on gay marriage is likely to be one of the defining issues of the 2012 White House race. It could be issue with which Obama rediscovers his crusading zeal, wins the moral high ground, recaptures the support of disillusioned gay and liberal voters. Cynics also suggest that it will boost gay funding for his re-election campaign.

In some respects, however, Obama is merely catching up with the historical trend towards lesbian, gay, bisexual and transgender (LGBT) equality. Same-sex couples are already allowed to marry in Canada, Spain, Portugal, Argentina, Norway, Sweden, Belgium, the Netherlands, Iceland and South Africa; plus some regions in Mexico and Brazil. Denmark is expected to legalise gay marriage next month. Within the US, gay marriage laws have been passed in Connecticut, the District of Columbia, Iowa, Maryland, Massachusetts, New Hampshire, New York, Vermont and Washington. A majority of Americans support equal marriage rights for all.

Obama’s move throws the spotlight back on David Cameron and recent wavering in the Tory ranks on this issue. The UK campaign for marriage equality has been spearheaded by the Equal Love campaign. This involves a twin-track strategy, both political and legal. We’ve succeeded in persuading the government to legalise gay marriage by 2015; although the Prime Minister is under pressure from a rump of homophobic Tory MPs to drop this commitment.

On the downside, despite his pledge of equal marriage rights, David Cameron is, thus far, refusing to end the ban on heterosexual civil partnerships and the ban on religious same-sex marriages by faith organisations that wish to conduct them.

As a back-up plan, Equal Love has filed a legal case in the European Court of Human Rights.

Four gay couples and four straight couples have applied to overturn Britain’s twin bans on same-sex civil marriages and opposite-sex civil partnerships. They seek full equality in civil marriage and civil partnership law.

Gay marriage is all about love. The love of same-sex couples is just as real, strong and committed as that of married heterosexual men and women. Prohibiting same-sex marriage devalues and denigrates the love of lesbian and gay partners. It signifies our continuing second class legal status.

Prohibiting same-sex couples from civil marriages and opposite-sex couples from civil partnerships is as offensive as barring black or Jewish couples. It’s a form of sexual apartheid to have separate laws for gay and straight people.

In a democratic society, everyone should be equal before the law. The Equal Love campaign wants civil marriages and civil partnerships open to gay and heterosexual couples, without any discrimination based on sexual orientation.

British public opinion is overwhelmingly in favour of same-sex marriage. A Populus poll in March 2012 found that 65% of the public agreed: “Gay couples should have an equal right to get married, not just to have civil partnerships.” Only 27% disagreed. This represents a steep decline in opposition to marriage equality.

Public support for gay marriage is double the support secured by the Tories in last week’s local elections. If David Cameron wants to win back lost votes he should fast-track legislation for equal marriage rights.

Peter Tatchell is coordinator of the Equal Love campaign. For more information about his LGBT campaigns and to make a donation, visit www.petertatchell.net.

A version of this article first appeared in The Guardian.