The engagement photograph showed Brian Edwards and Tom Privitere at their happiest, celebrating their love and upcoming marriage. Two years later, they were shocked to see it used in an advert opposing same-sex union.

"This case is about the defilement of a beautiful moment," say the New Jersey couple's lawyers - amid the typically dry language of a lawsuit lodged with a federal court.

Brian Edwards and Tom Privitere learned in June that a shot of them sharing a tender kiss had been used in a political mailshot, in a state far from their home.

"They use you personally to instil this fear of us and this disgust of a gay couple," Privitere says.

"You get so disheartened and it's really a painful feeling that makes you feel like an outsider. It makes you feel unequal."

Image caption A wintry woodland scene was super-imposed

The mailing attacking a Colorado state senator who had backed a same-sex civil union law was sent out by Public Advocate of the US, a small political organisation based in a Washington DC suburb.

Public Advocate describes itself as opposing abortion rights, same-sex marriage and "so-called 'Gay Rights'". Publicly available records show it had a 2010 budget of about $1m (£619,000).

Its single reported full-time employee, president Eugene Delgaudio, did not respond to an e-mail request for comment. The voicemail box for a phone number listed as Public Advocate's was not accepting messages.

Edwards, Privitere and photographer Kristina Hill last week sued Public Advocate, saying the group had used the photograph without permission and had violated the men's right to control how their likenesses are used.

You can definitely see that we're smirking in the photo Brian Edwards

"We're here today to stand up for all families," Edwards says.

"We want to spread the message that our family is just like other families and we want to be treated equally. We don't want this to happen to any other couple."

The two men met about 12 years ago in New York City, when Edwards was in university and Privitere was working as an actor. They moved in together in 2002.

In December 2009, Privitere proposed to Edwards - down on one knee, surrounded by friends and family. The following May they hired Hill to shoot photographs to celebrate the engagement.

"That was really just a fun day to go to our favourite places in Brooklyn and capture our favourite locations in the pictures," says Edwards, now a 32-year-old university administrator.

"It really was a wonderful culmination of a journey," says Privitere, a 37-year-old ticket broker.

"Growing up gay, you're not always feeling the most accepted, you're not always feeling the most welcome. When you know you find the person that you love and you're going to be with and you begin to celebrate and take these photos... it's really an overwhelming feeling of joy."

The day's outing ended in Brooklyn Bridge Park across the East River from lower Manhattan. Hill posed the two men leaning in toward one another and smooching.

Image caption Privitere, left, and Edwards, seek compensation for damage to their reputations and emotional well-being

"You can definitely see that we're smirking in the photo," Edwards says.

"When I see this photo I imagine myself making him laugh and holding his hand and saying, 'Don't move, just kiss me.'"

The men posted the photograph on their blog. In September 2010 they were married in a civil ceremony in Connecticut, and the following month held a wedding in Mexico. They returned to New Jersey and settled back into their lives.

On the other side of America last June, Republican Colorado State Senator Jean White faced a tough challenge from a rival in her same party.

Voters in White's district received a mailing showing Edwards' and Privitere's kiss, superimposed on a snowy winter scene, saying the image illustrated White's "idea of 'family values'".

The mailing gave Public Advocate's name and web address. The photograph was used in a similar fashion in at least one other campaign.

White lost her election.

Brian grew up being bullied Tom Privitere

The use of the mailing was reported in the New York Daily News. A friend of the couple saw it and recognised them.

"When I first saw the image I was completely shocked," says Edwards.

"By the time Tom got home I was really angry, then that morphed into heartbreak."

The men, and lawyers with civil rights organisation Southern Poverty Law Center, surmise Public Advocate found the photograph on the internet, and used it to avoid paying for a stock photograph.

Public Advocate has not yet responded to the lawsuit, filed in federal court in Colorado. Delgaudio told the Associated Press news agency that he was "looking into this".

The pair had grown accustomed to New York City's atmosphere of tolerance and were rudely reminded not everyone in America takes the same view of same-sex partnerships, Privitere says.

They were also distressed to think gay children and teenagers struggling to come to terms with their identities had seen the mailing.

"Brian grew up being bullied and it touches back on acceptance of who you are," Privitere says.

"They saw that flier and they thought, 'Maybe I never will have a family, maybe something is wrong with me?'"