...at least that's what this man claimed. I had let slip that I was leaving the religious scene and he said he, too, had realized that Mormonism was wrong... I thought we would have some common ground.

Nope.

He continued to explain that he believed the church had gone astray but that he believed the book of mormon and everything that Joseph taught. Then he announced his belief that Joseph never taught or practiced polygamy.
"Um... how do you figure that?" I asked. "There are plenty of period documents..."
"They were all written later than the fact," he said. "They only indicate polygamy if you try to twist them really hard. It's all retro-con. It was actually all Brigham Young."

What the heck? How does someone come up with that?

He may be a fellow traveler with, or influenced by, Rock Waterman, et al.

http://puremormonism.blogspot.com/2010/06/why-im-abandoning-polygamy.html

That's so odd because my good friend believes exactly the same thing! She and I stopped going fairly close to each other, but our beliefs are SO different. It makes talking about it really awkward. She thinks that Brigham young black mailed JS and hired the mob to kill him (insert eye roll here)...

This was standard RLDS doctrine until they overhauled their church and changed it to Community of Christ. Someone else may be able to fill in the details...

I told some mormon girl that I did not agree with polygamy and then she went on and said that they had to get married but didnt do anything sexual!!
Then, why would they get married? and she was all "to help out"

Omg, so naive.

Amazing how they strain to explain away polygamy but swallow the book of mormon whole.