Mr Marsden, the MP for Blackpool South, used an interview in his local paper to go on the record about his sexuality, and said that it was not a defining influence on his politics. Earlier, David Borrow, the 45-year- old Labour MP for Ribble South in Lancashire, also declared his homosexuality.
The men join a growing list of politicians who have come out of the closet. Chris Smith, Stephen Twigg and Ben Bradshaw are all openly gay, and the environment minister Angela Eagle came out last year in an interview with The Independent.
But the former Conservative MP and journalist Matthew Parris said yesterday that coming out was so routine it had ceased to be interesting. "Labour MPs seem to be coming out all over the place," he said. "Some of them are people we have never even heard of before but we are in a new climate of sexual liberation and it could well be a bid for some personal publicity."
Angela Mason, the director of Stonewall, said that homosexuality was becoming more accepted in all walks of life and that had filtered back to Parliament.
"People are just refusing to live their lives in the closet any more, and thousands are coming out in all areas and claiming equality," she said. "Parliament is simply reflecting those changes."
She suggested that a forthcoming vote on lowering the age of homosexual consent from 18 to 16 had precipitated Mr Marsden's and Mr Borrow's decisions.
"They both wanted to be able to speak in the debate and discuss the issue openly," she said.
Mr Marsden, a former historian and lecturer who won an 11,616 majority at the last election, said there was no reason why his constituents should not know of his sexuality.
"Many people in Blackpool already know - I believe my constituents have the right too. I have made no attempt to hide it or mislead people," he said. "This is merely the first time I have gone on record and spoken publicly about it."
He said he shared a home with his long-term partner, named only as Richard, in Brighton. They met 12 years ago in London.
"I'm a politician first and a gay man second. I am not a single-issue MP. Being gay is part and parcel of what I am, who I am."
His declaration would come as no surprise to those who knew him best, he added. "I don't lead a double life. Most of them know. Some have met my partner. But a politician stands for election, not their partners, or their parents. They have a right to lives of their own."
Even the Conservative party seems to be relaxing its stance on the issue of homosexuality, although no Tories in the current House have admitted to being gay. In the last parliament, one Tory MP, Michael Brown, admitted to being gay. He is currently writing parliamentary sketches for The Independent.
Environment minister, Angela Eagle, 37, (Wallasey): "I'm in a long-term and happy relationship. I happen to be with a woman."
Ben Bradshaw, 36, (Exeter): "Gayness is now mainstream: why behave as though we're still in the ghetto?"
Stephen Twigg, 30 (Enfield Southgate): "I think it is positive if public figures can say they are lesbian or gay."
Culture minister Chris Smith, 46, (Islington South): "Gay men are different - that doesn't mean we are not as valid as any other citizen."
David Borrow, 45, (South Ribble): "I think it is appropriate at this point to be honest with the people of South Ribble."