I think you make a good point. I neglected to look at Titus 1:5 which shows verse 6 is about elders. I also agree with your conclusion that scripture indicates some people in the Church had multiple wives (as well as slaves).
What is significant is Paul makes a distinction that lay people in the service of the Church (elders or deacons) must set a good example and that good example includes being the husband of "one" wife. Under Paul's definition a polegamist could never become a leader within the early church. Why would a true Christian want to be a polygamist knowing that he could never be in a leadership position? Where is his heart? Paul cleverly never condemns or offends anyone but backhandedly makes the case that polegamy is wrong.
If I recall, the gentile cultures of the time allowed polygamy. When they converted to Christianity, what would happen to this kind of a family? I believe those were the roots of its tolerence in the early Church.
I think if you were to look at Matthew 19:4-6, the will of God in the matter is abundantly clear:
He said in reply, "Have you not read that from the beginning the Creator 'made them male and female' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So they are no longer two, but one flesh. Therefore, what God has joined together, no human being must separate."
The above text is then re-iterated by the Apostle in his letter to the Ephesians:
So (also) husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one hates his own flesh but rather nourishes and cherishes it, even as Christ does the church, because we are members of his body. "For this reason a man shall leave (his) father and (his) mother and be joined to his wife, and the two shall become one flesh."
Again, we can see that the concept is one husband, one wife and that in marriage they become one flesh. If polygamy was within God's plan, all two, three, four, or one thousand wives would have to become one flesh with not only the husband but also with each other (otherwise they couldn't be truly one flesh with the husband).
And then the analogy St Paul points out is very apt in this discussion, as well. Christ only has one bride: His church. There are not many churches, but only one. Elsewhere, where the Church is analogized to Christ's Body, it doesn't refer to multiple bodies...just one.
In light of these scriptures (which I consider to be far superior as proof texts), it is apparent that any tolerance for polygamy must have simply been a grace afforded to converts from the gentile races.