If this group of folks arrive to populate the planet, why...

I'm honestly curious about that. The only thing that's occurred to me (and it's not very likely) is that they've done some kind of intense DNA/genetics analysis and their DNA will go into the mix artificially to create as genetically diverse a population base as possible.

But even that seems really unlikely and impractical because it would seem more likely that they're there to set stuff up and make it safer for a following wave of colonists, so they're not growing a population from the ground up. So maybe the couples aspect is to help keep people on a long assignment from suffering psychologically (although, you could joke that marriage is stressful enough). Plus, from a script standpoint it creates a quick way to communicate emotional attachment.

No idea. :-) But I'd like to hear the explanation!

Because in the liberal Hollywood, brains don't function.