As many here predicted — the “gay’ marriage advocates who promised that they wanted nothing to do with polygamy or pedophilia have turned to the next attacked on civilized society.
The next assault on the family I could see would be two gay men “marrying” their surrogate or breeder. She gets to have a baby they primarily raise, there are fewer legal wranglings than getting a surrogate to give up her rights or pay her, and the men can divorce her when they no longer want her around or leave the child with her when they want to go out. And they’d argue that it is creating a family like ours, just with an extra adult.
The other oddity would be the two gay men and two gay women gathered into an extended family, with the men fathering children artificially with the women (I’ve hard of it) and gaining rights via marriage while remaining with their same-sex partners.
But the Heinlein views of polygamous families and line marriages as happy, supportive places are purely fantasy. And “open” relationships make everyone jealous if not physically ill, in a way that a committed marriage will not.