I don’t know much about the UMC, but I think it stands to reason that a homosexual relationship with a married person is adulterous. Any sexual liaison with a person other than your spouse constitutes adultery.
Yes but the UMC will have to recognize the union as legal in the yes of the church.
The UMC doesn’t recognize polygamy even if the people are legally married on their country. So a person practicing polygamy cannot be ordained.