The situation posed was the man being jealous of his female partner.
If they two were suspicious of each other, I'd agree with 'they'. I also, without saying it, said that the woman should stay home and not be out and about.
I'm a traditionalist regarding marriage and generally don't bother myself with asking myself a question I already have answered.
As you wrote, “Marriage unites two as one,” and if it takes two, then “way back there” it was “they” who didn’t unite, not just “she”.
She could not have united in marriage without him anymore than he could have united without her. The fact that one may have been willing to unite and the other was not willing does not change that. That the situation you posed (and that I did not catch) was the man being jealous of his female partner does not change that. I say “you posed” because the article talks about partners, mates and friends unless I missed something.