Khan was married. Who performed the ceremony?

[Guy Gardener]

What if Khan lied?

The ear slug might not have been an accident.

She got tired of his shit, and he labotomized her.

Or...

To maxamize the proliferation of supermen, all the women and were fed earslugs, and produced babies like brain dead factories.

 

[JonnyQuest037]

Yet Kirk had no issue in letting her stay with khan and with no way ever to change her mind.

She helped Khan take over the Enterprise, aiding in the attempted murder and torture of the crew. I sure wouldn't want her staying on my ship.

And Kirk gave her a choice: court martial or accompanying Khan to Ceti Alpha V.

KIRK: Yes. I understand. Lieutenant Marla McGivers. Given a choice of court martial or accompanying them there.
KHAN: It will be difficult. A struggle at first even to stay alive, to find food.
MARLA: I'll go with him, sir.
KHAN: A superior woman. I will take her. And I've gotten something else I wanted. A world to win, an empire to build.
KIRK: This hearing is closed.

Kirk was more than generous with her, I think.

And despite what Khan thought, it wasn't Kirk's fault that no one from Starfleet ever checked in on Khan's colony. (Or maybe Starfleet just checked on them from afar and never revealed themselves to Khan. The Khan we meet in TWOK is largely insane with a limited perspective and should not be taken as a 100% reliable source.)

 

[jackoverfull]

She helped Khan take over the Enterprise, aiding in the attempted murder and torture of the crew.

after having been brainwashed, yet she still turned on khan eventuality. The same khan who would now have power of life and death on her for the lest of her life.

I sure wouldn't want her staying on my ship.

Sure, but there are rehabilitation facilities in the federation. We saw a couple of them.

 

[Guy Gardener]

She wanted to write a book and then do Ted Talks.

It was her Road back to being someone.

The idea, that no one was coming back to check on them for 15 years, and by accident, at that, was unthinkable.

 

[Roundabout]

It’s clear Khan was married presumably to the crew woman who left with him (McGivers
Iirc).

So who performed the ceremony and was qualified? Under what governing law?

Have you ever read Milton?

Khan was marooned in what turned out to be a godforsaken planet. But it was Khan's godforsaken planet. His planet, his rules. If Khan wanted Marla to be his "lawfully wedded" wife, so shall it be.

 

[Kor]

Or if they didn't bother with formal legal paperwork and licensing in Khan's dominion, it could have been what is often known as a common-law, informal, non-ceremonial, etc., marriage. They declare that they are married, and everybody else accepts it.

Kor