Some Christian sects have followed the practice of Polygamy...
Is there anything in the Bible to support that? How did they justify that practice?
Does it have any merit?
No mainstream Christian tradition, communion, or denomination accepts polygamy as valid practice. It has always been considered invalid within Christianity.
There is no explicit condemnation of polygamy in the Bible. But Christians have always recognized that marriage, of sacred partnership, is valid only in the context of monogamy.
In the Old Testament we see that the ancient Israelites, like most societies of the time, had polygamy. It's not as though every Joe, Tim, and Hugh had multiple wives. But if you were wealthy, if you had power, and if you wanted to establish strong ties with other families, tribes, clans (etc) then marriage was a way of securing relations with other groups. From tribal society to kingdoms, marriage as a securing bond continued to be practiced. And so kings would have multiple wives, priests and other powerful men would have multiple wives--securing relations with other kingdoms, or other powerful families, or etc.
This was normative in the ancient world.
While sometimes God's instructions to the Israelites explicitly violates the norms of ancient society, for example "You shall have no other god" is an explicit violation of the common belief and practice of polytheism. "No graven images" is a violation of the common practice of making idols, images of the divine for the purpose of worship.
But other times, God's instructions tend to be more subtle in their subversion. There are things which are not outright condemned, but which are curbed. So, for example, there are rules about what happens if someone takes another wife--there is no explicit condemnation of this but neither is there any explicit permission for doing this. Rather, it speaks of this as something that could happen, and then there are instructions that put barriers around it--things to do and things not to do. So, if someone takes another wife, they owe their new wife the same obligations and responsibility of their first wife; and vice versa. You did this, now you have to take full responsibility for what you've done.
Polygamy is never given sanction; though it is never explicitly condemned. At least in the Bible itself.
However, Christians have always maintained that the only valid marital union is a monogamous (and monoandrous for that matter) one.
The Church, therefore, has historically always refused to sanction the taking of more than one spouse.
-CryptoLutheran
