Barack Obama has become the first American president to back same-sex marriage. Mitt Romney, the Republican who's set to challenge him for the White House promptly said he was against gay marriage. The politically charged issue still divides opinion in the United States. Same-sex marriage has been passed in 8 states and is active in 6 of them. But is still not recognised by the federal government. 31 states have passed constitutional amendments or legislation to prevent same-sex marriage. George Alagiah was joined on the GMT programme by Andrew Marsh from the pressure group Christian Coalition and by the gay rights activist Peter Tatchell.