"Big Green Man" or "Is Sex the Final Frontier"?
One of the difficulties that grows out of taking Science Fiction seriously to any degree - especially the Star Trek Universe - is the concept of interspecies breeding. I mean, really, can you imagine some of the difficulties that might arise between the different Humanoid forms. And there's another point: why Humanoid? Why not Klingoid or Vulcanoid? Why does the Federation insist on relating everything to the Human form? Rather than, say, the Vulcans who, after all, have had the strongest philosophical effect on Federation thinking.
The whole concept is so Homo Sapien centered when you think about it. But, I digress.
How can humans and non humans mate? Think about Spock for example. Never mind the differences between his father and mother's sex drive. She's a normal human female and likes sex about once a week, or maybe once a day, or maybe once a month. Oh, what the hell, more than he does: Every seven years.
Imagine what it would be like if he suddenly gets the urge and she has a headache?
Or that he is two to three times stronger than a human. So, having got the timing right, he crushes her to death during intercourse having gone boldly where no Vulcan has gone before.
Or she survives with only a few broken bones and a bad concussion. And, if the Vulcan male sperm is compatible with human female eggs, what female egg is going to survive an attack by tens of thousands of stronger-than-average micro-tadpoles?
But, suppose we get through this and everything works to the point where the cells divide properly and a new life is created. The other point that crosses the mind is the unborn copper-based Spock, floating around in his mother's body trying to take nourishment from a Iron-based blood supply. Death by starvation. And - of course - being mildly telepathic the unborn Spock would let Mum know that all is not well. She may survive physically but I can't imagine her surviving mentally or emotionally.
Or - should these matters be overcome by advanced Vulcan science - as he decides it is time to be born, he smashes his mother's pelvic bones on the way out. This is one, STRONG baby we are talking about. Of course, the Vulcan Medical team could bring the baby Spock into the world by Caesarian section, but Caesar = Rome = Romulan, which of course would never do!
Actually, under Canon law, the union of two species could be classified as a form of bestiality and therefore, not permissible, so the whole thing is moot anyway.
Ah well, Twenty third Century Medical Science triumphs over Twenty First Century Skepticism.