The only prohibition to "polygamy" in the NT is found with regards to elders/overseers and deacons/servants:
1 Timothy 3:2
A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach;
1 Timothy 3:12
Let the deacons be the husbands of one wife, ruling their children and their own houses well.
We also see a reference in Titus regarding elders/overseers:
Titus 1:6
If any be blameless, the husband of one wife, having faithful children not accused of riot or unruly.
I believe that this is because God's perfect will is monogamy. It should be noted that as the church expanded into foreign lands where polygamy was legal, men with multiple wives were welcomed into the church, as were their wives and children. However, these men were prohibited from serving in leadership and were encouraged to teach their children to live monogamously.
So, the church has never really condemned polygamists as being outside of the scope of the Gospel. They are just unable to serve in ministry. Monogamy was to be introduced as a philosophy for the next generation. Monogamy reflecting one Lord and one faith (church).