Bill entered Polygamy for money and sex
I know Bill acted like it was his calling to live the Principle but I don't think so. I think he convinced himself of this.
He was happily married to Barb and he actually spoke out against Polygamy. Barb got sick and Nicki helped Nurse her back to health. He saw an opportunity to start a business by marrying Nicki and getting a loan from Roman. I think he even admitted this to Barb and said he talked her into it out of selfishness.
Margene started out as a straight sex toy for Bill. If you watch Big Love the Beginning, you can see why Bill was blinded by lust for Margene.
Favorite movie of all time - SCARFACE