Extra Parent Conception: Leaving aside for a moment the issue of species which naturally have more than two sexes (which is extra from our point of view, but not from theirs) plus the issue of cladism and other deliberate modifications, this is the sort of thing which the same reproductive technology that conveniently enables Homosexual Reproduction makes downright trivial, conveniently enough for the polyamorous married.

There’s a reason why current birth documentation *there* registers bio-parenthood by “percentage of genetic complement”. (Not that either that or “incubator” is used for much but medical purposes; there’s a reason why it records three different kinds of parenthood, too.)