Alright, now we’ve all seen alien couples in science fiction.
Captain Kirk case and point, I suppose you could probably count jabba the hutt and princess Leia too if you really want to make that leap of imagination about what was going on in between the empire strikes back and return of the jedi.
Of course this has already been spoofed to hell with Futurama’s kif and Amy, but really, and I know this is obvious for many people, it would be pretty much impossible for life forms that unrelated to each other to have offspring.
Here’s looking at you Spock.
And I’m not even going to mention the Doctor Who TV movie.
I liked your take on it Paul McGann but being half-human just doesn’t make sense given the history of the time lords etc.
But to demonstrate this, first think about how hard it would be to reproduce with a gorilla.
They’re pretty closely related to us and already there are complications.
Now move on to crocodiles.
Now move on to mushrooms.
Now move on to flagellated protists.
And slime molds.
Now you’re down to bacteria, you may have noticed how much more difficult its getting each time to mate with these creatures.
But once you get past that, to viruses using double-stranded DNA it’s even more difficult.
And even if aliens had a very similar morphology to us, the biochemical differences alone would make having sex with an alien even less plausible than having sex with say… a bacteriophage.
But moving past that even, when you look at how many different ways life on earth has worked out how to reproduce, it’s unlikely that beings from another planet would be multicellular, intelligent, human-like and using the same penis/vagina method our kind have settled upon.
For all we know you could pull down the doctor’s pants and find a flower down there.
It’s just as plausible.