I always thought the "they came together" had subtle sexual implications as well.
One of the things I like about the way Proulx uses language is that her word choices often yield more than one interpretation, but regardless of how you choose to read a line, it still fits. Each of the interpretations adds a different, but still appropriate, dimension to the meaning.
The double entendre of "they came together" gives it a gritty, crude tone and at the same time foreshadows how their relationship is going to progress.
"They came together on paper" conveys the notion of marriage as well, and it does all that at the same time "they came together on paper as herder and camp tender..." conveys the literal meaning of the sentence while introducing a hint of irony that this is not the way things are going to be. It's the unusual choice of wording and the order in which the sentence unfolds that allows these multiple meanings.
I think this multiplicity of appropriate meanings is what makes the story feel so rich.