Of course I believe that religion existed with Adam.<quoted text>
To find the origins of marriage, look at continuity of family, property, tribe and political influence. Socially, those things all predate religion, unless you believe that religion is older than mankind. shrug. Not much help for that.
The concept and practice of "priesthoods" are not confined to the LDS, or even Christianity, and like any other power structure it is more often peppered with segregation and elitism than not.
I'm only speaking of the priesthood in the Church of Jesus Christ of Latter-Day Saints. I exclude the rest with my comments.