Well in Genesis God says that marriage is between a man and a woman, and on the sermon on the mount Jesus says that simply looking with lust is adulterous (applying to unmarried couples). So, if men and men or women and women cannot get married, and have homosexual feelings that they don’t work to suppress, it is adultery and a sin. This also applies to heterosexual couples that look with lust before marriage and don’t work to suppress it
