I'm pretty sure I introduced him to the idea of Biblical plural marriage, but I don't think they're too overly concerned about the Biblical aspects of it. My dad is... only sort of Christian. He's in the "believes in God and Jesus" grouping, but doesn't have what you'd call a doctrine or anything.
Man Jason, glad that you've made progress, but yeah, we've had much better reactions. Of course, I haven't actually taken a second wife or anything so there is the potential of subject to change, but I don't think so. There was a "strong potential" that just couldn't accept the Biblical basis of PM (or go against her family's wishes) that was in our lives for quite a while, and my mom (who was the closest to "against pm" from our parents) really did great getting to know her and hanging out with us all and including her in stuff. Even like buying her a Christmas gift and picking her up candy for Valentines day, so I'm sure my mom would continue to be loving and accepting if I am ever able to add another wife.