Polygamy isn't allowed, but there are people who have got more than one wife. In my locality if someone marries second wife without getting proper divorce from the first wife, then he will be kept away from the church activities. Once after making everything legal, then only they were given rights to be a part of the church community.
If that's the case then for me it's immoral maybe not from the eyes of human but definitely in the eyes of God. Actually, the moment you broke your marriage is already a huge sin, what more if you not only left yout partner but also find a substitute for her. What such a disgrace.