A 2 sets of “lesbian and a gay man” get married at the beginning of the story because they each want to have children, but do not want to do it on their own. Each couple promises each other that they will not start dating other people until their children are old enough to understand their choices.
The two husbands of the couples meet at work some years later and begin seeing each other. It comes out (yes, pun intended) that they each are married to a lesbian, so they try setting their wives up with each other. The couples agree to meet on a night where they can send their children to their grandparents house for the night.
The kids are old enough to sneak out (early teens/tweens), so they follow their parents to the place they are supposed to by meeting up at. The two groups of kids meet and figure out that their parents are seeing each other. Each kid is horrified that their parents are cheating on each other together and plan to break them up.
The two wives hit it off and the gay and lesbian couples grow more serious. The kids pull awful pranks to try and break them up, but not matter how hard they all try, their parents are still in love. The husbands convince their technical wives to come out to all of their kids at the same time one night.
When the dads tell the kids that they are dating and the moms tell the kids that they are dating, the kids tell them that they know and tell them that they don’t want their parents to break up. The kids don’t want to have to go from house to house just to see their parents.
As a compromise, the two couples decide to move in together and raise all of their kids together.
This can lead to a sequel/series about the mischief the kids get into and the dysfunction of having too many people in a house.