Every religion have their own beliefs and doctrines. It is good for we Christians to have full knowledge and understanding about the doctrines in the bible so that we don't get deceived by false teachings.
Marriage is an honorable thing before the sight of the Lord when done rightly. Marriage is an institution designed by God. He created it to be a union between one man and one woman. It demand faithfulness. When more than one spouse is involved then faithfulness is missing because faithfulness is supposed to be directed to one person alone.
Beyond the physical knowledge we have about marriage it also has a spiritual significance attached to it. The union between a husband and a wife is likened to the union between Jesus and the church.
The Bible refers to Jesus as the husband and the church as the bride. That's why in Revelation we see where there's a wedding. That wedding refers to Jesus and the church. The Bible also tells us that God is a jealous God and so we should have no other gods except Him.
This is taking about faithfulness. The same thing applies to the physical marriage we carryout here on earth. The same way we are faithful to only one God is the same way we should be faithful to only one spouse. This is how God desires that marriage should be.