Group marriage in SF

Alternative Family Arrangements

Group marriages, poly/nonmonogamous, sexuality separated from childrearing, etc.

Block, Francesca Lia. Dangerous Angels: The Weetzie Bat Books (1998). This collects five short novels in one book. Weetzie Bat (1989) - Witch Baby (1991) - Cherokee Bat and the Goat Guys (1992) - Missing Angel Juan (1993) - Baby Be-Bop (1995)
Dorsey, Candas Jane. Black Wine
Grae, Camarin. Stranded (In the protagonist's home world, 3-way relationships exist and are respected, although it is acknowledged that they can be difficult. On Earth, 3 of the characters in this novel decide to attempt such a thing also.)
Heinlein, Robert. Friday and others. Heinlein mastered the art of coming up with reasons and social forms for women to get it on with each other and men, but for men to stay largely manly and heterosexual.
Le Guin, Ursula. The Ekumen / Planet O stories: "Mountain Ways" and "Unchosen Love" [four person marriages are the social norm]

Brantenberg, Gerd. Egalia's Daughters
Bujold, Lois McMaster. Ethan of Athos
Dodderidge, Esme. The New Gulliver
Jones, Gwyneth. Life ("Spence")
Wen Spencer
Tepper, Sheri. The Gate to Women's Country. At 5, boys are sent to live with the men in the military camps.
Varley, John. The Gaean Trilogy, v. 3: Demon [not a social norm, but an interesting daddy role]
Wells, John J. [pseud. for Marion Zimmer Bradley & Juanita Coulson], "Another Rib," The Magazine of Fantasy and Science Fiction (June, 1963) [when an all-male crew returns from outer space to discover Earth destroyed, they have to figure out how to reproduce with each other]