Jesus said marriage is between a man and a woman. No other sexual relationship is permitted. It is immoral.

Matthew 19
4 “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”