Kirk and Spock are sent to Uriman V to smooth things over after a premature First Contact with an alien race.  Things start off well...and then go horribly wrong...Certainly not meant to be slash, but with enough alcohol I suppose anything's possible...

In the real world, it has the force of scientific law: only the most similar species within a genus can successfully mate, and even then the offspring will almost always be sterile.

Yet in Star Trek, hybrids have been with us from the very beginning.

So how is it that beings that evolved on completely different worlds, some of them with physiology fundamentally different from Earth humanity, manage to have healthy children together? And how is it their children seem to expect the same?

Here's how it's done in Sigils and Unions...

(Written from an out-of-universe perspective, but with references to Sigils and Unions/SigCat events. Please note that the PCE theory is likely unknown to the public due to the political implications seen in "The Chase"; however, its consequences on domestic life are well known.)