Homosexuality is not natural. The natural design is for man and woman. They can conceive children. It is not natural to couple male with male and female with female. It would be like trying to fit two screws together or two nuts together and then say, "See, it's natural for them to go together."
1 Cor. 6:9-10 and
Rom. 1:26-28 has much to say about the situation. The Bible, of course, condemns homosexuality. It is also not rocket science to discern that homosexual marriage is also condemned. But our society does not rely on the Bible for its moral truth. Instead, it relies on humanistic and relativistic morals upon which it builds its ethical structure.