Stephen Ponder, 28, a special constable, and his partner Ivan Sigston, 43, a Hampshire police dog handler, began an affair after they met when still with their previous partners.
Now Mr Ponder is seeking a divorce from cleaner Ian Briggs, 49, while Mr Sigston is also divorcing former partner Steve Aylett, 38, a banker.
Earlier this week it was revealed that Mr Ponder's sister, Lorna Bradley, had given birth last month to a baby, named William, who was conceived using Mr Sigston's sperm. The two men are now bringing up William at their Southampton home and plan to legally adopt him.
The two gay couples are reported to have met several years ago through a gay networking site and their shared interest in dogs.
After becoming close friends, both couples were witnesses to each other's civil partnership ceremonies in 2007. But neither partnersip lasted more than a few months and Mr Ponder and Mr Sigston began a relationship. However it was disclosed today that Mr Briggs is resisting Mr Ponder's divorce application.
He was quoted as saying: "It is clear they were having an affair behind our backs. The four of us became close, but I had not idea they would end up running off. The weddings must have been a sham as they became a couple within months."
He was quoted as saying: “I've put it behind me and want to look ahead.''
Mrs Bradley, 31, who is married with three children, is said to have volunteered to have the baby for her brother and his partner out of “sisterly love” and no fee was paid. Because Mr Sigston was not registered as the father on the birth certificate, he and Mr Ponder have to adopt the baby, who is now around three weeks old.