R.C. Sproul took a question from a pastor on his radio show that interested me quite a bit. The pastor had a new convert in his church that had been a polygamous Mormon. The man had left Mormonism but not any of his three wives. The pastor was curious how he should go about disciplining this man and if he should be encouraged to leave his two additional wives.
You can hear Sproul’s answer here. Start at the 9:00 mark.
I expect Christians to encounter more situations like this one. And the dilemma isn’t just for polygamist. What should two married women with children be counseled to do if they join a church? Two married men? How about a man who divorced, as a believer, for unbiblical reasons and remarried?
I’m sure the situation is semi-regular in the LDS church. What must a polygamous man do to be baptized and enjoy full communion in the LDS church?
LDS & Evangelical Conversations 
Like Dr. Sproul I think our response has to be pastoral. I think the unresolved question is how to do this openly within a congregation. Do you explain the situation and the logic openly? What will visitors to the church think when they see a man with three wives on Sunday morning?
I think your hypothetical situation with a civilly married homosexual couple brings even more questions. How do we ask the couple to continue going forward? What is our stance once the children have been raised?
I grew up LDS in Tennessee, so no. Not regular at all.
But I bet the CHI says what to do about it.
FWIW, I served my mission in So. Utah and had some experiences with polygamy. First, I never encountered anyone or have heard of anyone attempting to keep his/her polygamous relationships and also join the LDS Church. It might be because I’m certain they would be asked to end the marriage relationships that were not recognized under the law, while still maintaining child support responsibilities the same as any other absent parent that qualifies.
In general, the LDS Church takes the baptizing of individuals who have a history with polygamy very seriously. The people I encountered had to have multiple interviews that looked like this – District Leader > Mission President > Apostle. That’s right, every single man and woman had to meet with (at the time it was always Elder Wirthlin) one of the 12. For context, the average baptismal candidate just meets with the District Leader. Some ex convicts of serious crimes had to meet with the Mission President.
Also, out of all the people I met with a polygamous background (20), not one of them were men with multiple wives or children. Almost all were women with children or 18+ men who had never married.
Polygamy in Utah is a very strange animal. I met an active LDS man once in Vernal, who one day felt “called” to take on another wife. His lawful wife supported him and they were consequently excommunicated. In Manti, I heard stories about members of the “TLC” who had lied in their baptism interviews, made it to the temple for their sealing one year later and then immediately re-engaged their polygamous lifestyle. I met a woman who fled Colorado City with her 6 children, saw herself and all her 8+ children receive an LDS baptism, only to lose her life to cancer. Does the polygamous father now get custody of the children? I left before that saga ended.
That is really interesting. Thanks CJ.
In a context where polygamy is not legal I think most churches would prescribe the same route; support the kids and stop sleeping with the mistresses.