Inter-species relations.

[Arpy]

The Protohumanoids shouldn't matter. Different species can't mate with each other. If humans and aliens were that genetically similar to each other, it shouldn't have taken the ancients spelling it out for them for peoples of 24th century sophistication to figure out that they're related.

I think they're very dissimilar genetically, requiring medical assistance to procreate. I like to think they also have different plumbing down there - just to up their alien-ness. Heck, they get more alien every time real world make-up techniques make it feasible. But about the plumbing, I bet they have all sorts of lingo for all sorts of different interlockings and pleasurings that must go on between different pairings.

 

[The Laughing Vulcan]

My own pet theory is that it's a combination of things, predominantly the Preserver/Seeder/Progenitor Intelligent Design populating the galaxy with humanoids...

Plus an Augment plot to take over the universe through genetic engineering. I've always seen the Eugenics wars as a war of improvement, with various factions unleashing retroviral bioweapons to reengineer the human race to their own vision of perfection. Just as the Klingons had their foreheads flattened by an Augment virus, I'd like to think that humans re-engineered themselves to make them more biologically open to accepting superior DNA from alien sources, to automatically assimilate what's better in other races and thus improve the species.

After the Eugenics War, and when Cochrane achieved warp flight and started the exploration of the galaxy, this and other disease vectors hitched a ride on the first ships, and started infecting and altering alien species as well.

It's why it gradually gets easier with time, as more species are infected and their DNA subtly altered. It's why Trip and T'Pol's baby wasn't viable, probably why Spock would have been conceived with some medical intervention, and why by the 24th Century, different species were mating like rabbits.

 

[Rett Mikhal]

I always assumed the Preservers and Ancient Humanoids just did similar things, and were not the same. The Ancient ones lived millions of years ago, before any other intelligent life in the universe evolved.

With their intervention, inter-species relations are possible, but without that, there isn't a snowball's chance in hell. The human form is, thankfully, unique. I refuse to believe any other life form could be humanoid; the universe is not so badly designed.

 

[Myasishchev]

Not to sound crude, but the basic sexual act may even make things incompatible. It seems that Vulcans and humans are anatomically and physiologically similar reproductively, if Trip and T'Pol can do it (so to speak). The same is true of humans and Klingons, humans and Trill, even Klingons and Trill.

But there must be some species in which the mechanics of reproduction preclude producing offspring.

Indeed.

Without mentioning interspecies breeding (it's stupid, stupid, stupid! okay, without mentioning it at length), it's not entirely likely that two species from entirely different biospheres would have physically compatible organs. Or that, if they technically could fit bits, it would be at all enjoyable or fun, or even that they'd be able to keep your lunch down.

I mean, it's not like weiners come in standard shape on Earth. Ever seen an echidna erection? Or a duck dick? Or a whale wang? It's horrorshow. Nothing that you'd want to lick, even were it attached to an otherwise attractive human(oid).

On the other hand, presuming that one is (pathologically?) intrigued by the alien's reproductive tracts, we have seen genital reconstructive surgery done quickly and easily in "Profit and Lace." So they've got that going for them.

The upshot is, no one here really wants to watch a 24th century porno.