Monogamy is simply neo germaine to homosexuality. Monogamy is the system suited for rearing children. Homosexuals do not have children of their own unless they buy them or get them unnaturally.
Homosex is unnatural use of sexuality because it is not productive and involves only one's self and "partners." It is a closed system that shuts out society and God. It is sinful.
If one is Christian then one accepts that God's creation is run by logical rules. The sexual act normally performed, results in continuation of humanity, the bringing into the world of more souls. One cannot be a flouter of rules and actually consider himself a practicing Christian except in that he may believe that it is true but that he chooses to contravene the truth. Then he could, perhaps, be sort of a Christian if a perverse sort.