Why did they make Molly Single?

Vulture's Matt Zoller Seitz talks about the "same/different" vibe that permeates the show in his review of the pilot episode:

You could look for one-to-one correlations between the movie and the show, but as soon as you find one that feels close, series creator Noah Hawley severs the connection. The pregnant officer played by Frances McDormand at first seems to have been split into two characters, a sheriff played by Bob Odenkirk, and his deputy, a gung-ho patrol woman (Allison Tolman, one of the best out-of-nowhere acting finds in my lifetime as a critic), but by the end of the pilot you realize, delightedly, that the script was going someplace else entirely.

...

[The pilot] ... develops a tone that could be described as Coen-esque, but does so organically, in a way that assures you that it will become its own thing. The next three episodes get incrementally weirder, stronger, and more original, to the point that you forget to measure this Fargo against its namesake, or against any of the Coens' masterworks...

It's a good read, one that suggests that creator Hawley is definitely trying to distinguish his characters from the movie. Each one evokes a familiar feel and then subverts expectations. In Molly's case, it's by being a strong, smart female deputy... but necessarily single and childless to distinguish her as her own person and not a Margie clone, at least physically.

Also:

"Eating the Blame" (season 1 episode 4, wiki) establishes world continuity by having Oliver Platt's Stavros discover the satchel of money buried by Steve Buscemi's Carl.

If Fargo the TV show is happening 19 years after Fargo the movie, then it makes even more sense for the characters to be different.

It's more than an homage but not quite a reboot or reimagining. Almost a sequel.

You might be wrong in thinking "Molly is clearly an adaptation of Marge Gunderson from the movie". Noah Hawley intended to make not so obvious that Molly is an adaptated Marge.

From this interview of Noah Hawley.

And then, as far as “Marge” and “Molly,” I knew that if I started the show with “Molly” as the chief of police, everyone was going to make a direct comparison to Frances McDormand and no one could survive that because Frances’ performance was so Oscar winning and iconic. So, I snuck “Molly” in through the side door. I created “Vern,” and I gave him a pregnant wife, and my thought was well the audience will go oh, I see what they’re doing, they just switched this and now the wife is pregnant but he’s having a baby, and then I kill him off and “Molly” has been introduced through the side door as a sidekick, so suddenly you realize only in Episode 2 that she is actually the star of the show, but at that point, you haven’t judged her against Frances McDormand. So, you’ve formed an opinion on her based on her performance versus based on somebody else’s performance.

Also, Molly had to be single, so she would be open

to start a love relationship with Gus.