"Nobody's gonna respect us if we have a gay homosexual boss sitting down discussing La Cosa Nostra business," an informer told the court on Wednesday.

John "Johnny Boy" D'Amato, head of the DeCavalcante family, the biggest in the state of New Jersey, was shot dead in 1992 after it was rumoured that he was having relationships with other men.

When a DeCavalcante soldier, Anthony Capo, heard the tale from D'Amato's girlfriend he was incredibly angry.

"She told me John D'Amato and her were going to sex clubs in the city, swapping partners and John was engaging in homosexual activity," he told the court. "It shocked me ... he couldn't be acting that way - he was a leader of men."

Early in 1992 Capo and another man, Victor DiChiara, picked up D'Amato from his girlfriend's home. As D'Amato sat in the back of the car, Capo turned and shot him twice, and when he continued to move, twice more.

In mafia culture there is a strict taboo against homosexuality.

"It's all predicated on male prowess and violence," says Clare Longrigg, author of a forthcoming book, No Question's Asked - the Secret Life of Women in the Mob.

"They all have wives and they all have at least one mistress. If they are gay then they would have to keep it incredibly quiet."

So strong is the prejudice that even after the family's leaders had approved D'Amato's murder they had to be careful how they went about it, lest word of his sexual orientation leaked out.

Normally they would have had to confer with the heads of other crime families - the commission - before executing someone as senior as D'Amato. But to share the news was considered so humiliating that they preferred to break mafia law instead.

"We knew we'd have to sneak him - kill him without permission," Capo said.

He was giving evidence at the trial of Stefano "Steve" Vitabile and two other mobsters charged with racketeering, murder and conspiracy.

Earlier, Capo told the court that he was inducted into the mob by dripping blood from his finger on to the picture of a saint, which was then burned. Then the family "counsellor" recited in Italian the vow that Capo would burn like the saint if he informed on his brothers.