While mating customs vary among different planets, the only legal arrangement that the Federal government recognizes in regards to the creation of families is the Procreation Contract. This contract is written and signed in addition to whatever local customs and/or laws require. The details vary but the basics is that the happy couple, or however many participants there may be, agree to produce and care for children. Usually the number of children and time they’ll let them act as dependents is specified, but it may be left open.
Contraceptive technology is near perfect with nanotechnology, and in any case many who aren’t hardline Noospherists undergo sterilization procedures at puberty, so family planning is relatively easy. Their progeny are almost always conceived in a petri dish and gestated in a Synth-Womb to optimize genetic profiles anyways. This habitual use of technology also enables same-sex couples and multi-partner groups to produce offspring that share genes with all their parents via a variety of different techniques.
The majority of Federation citizens grew up with the idea that they’re going to live an extremely long time, if not forever, and rarely believe in the notion of a romantic relationship lasting for eternity. Newly-admitted former outworlds frequently end up with entire legal firms dedicated to canceling contracts signed by lovesick ex-mortals. A more typical term is twenty-thirty years, sufficient time to raise one-three reasonably spaced out siblings. Once that term is up the participants may break up, chose to remain together in a non-breeding relationship, or renew later.
Another thing about Procreation Contracts that is hard for outworlders to comprehend is that it does not require any sort of sexual exclusivity. The contract completely separates the acts of sex and reproduction. Some groups do write up separate contracts for fidelity, but most leave it at the level of unwritten rules. And that isn’t even taking into account the various local customs that the Federation largely leaves intact.
The high families of genus Argentum have their own special contracts, “Pedigreed” Procreation Contracts are approved by the eugenics board to limit the deleterious effects of inbreeding among the Federation’s political elite. The children born of Pedigreed contracts are entitled to shares in Psi-Comm and recognition in the social circles that are a necessary part of a lengthy career in politics or bureaucracy. Family members can have children through unapproved contracts, but such progeny lack recognition.
Roughly half of all Pedigreed contracts actually are not drawn up by the signatory parties, they are arranged by other relatives in pursuit of some eugenic goal, such as the telepath breeding program, or in exchange for political favors. While the couple have the right to refuse to sign these contracts many elites have had mates they did not particularly like from the get-go, the fact that they aren’t even required to have sex helps smooth things out. There is some concern about the well-being of the resulting children but many families at that income level hire caretakers anyway, and even if the biological parents mated on their own volition the kids often feel closer to the hired help.
After the Plagues and the dispersal of the arcology populations to the countryside it became more common for small groups of friends to buy plots of land and build on them, oftentimes members of these groups mated with one another. However, disputes arose often enough that a standard contract to share housing arose in which the signees agree to pool resources for the maintenance of the grounds and welfare of those living on them. These household contracts are increasingly written alongside procreation contracts, especially in the colonies. Sociologists have speculated that without this addition to the Centauri culture the integration of the territorial clan-based Ceti would have been nearly impossible.