"You're going to hell" You hand them the Word, and say "Prove It ?"
The problem for them is they can't, not unless they do a lot of twisting and turning, taking scripture out of context, or not interpreting words correctly. As a former pastor (Mennonite) i can say that most ppl who sat under my teaching, never bother to look, much less study their Bible 2 Timothy 2:15, preferring to be spoon fed by a minister, who could say anything and often regurgitating man made dogma's.
By the time i reached Bible school and seminary, I realized a lot of church teachings were wrong, mostly to keep the money rolling in, and keep ppl in line. While i got excommunicated for my trouble in telling congregations the truth of God's Word, I have no problem justifying real biblical Christianity with Polyamory, which i believe the Word of God teaches and approves of.
For the Christian who truly desires to study what God has to say about Poly relationships, here are 3 links: