Some claim Jesus never said anything about homosexuality and therefore is neutral on the topic. Not true. Jesus was unequivocal in saying that to understand marriage and the sexual union, we must go back to the beginning and see how God created humanity and to what end. (See Matthew 19 and Mark 10.) Jesus holds up the creation story in Genesis not as a quaint Sunday school lesson, but as authoritative — reminding us that God created each of us male and female, each for the other. And the sexual union that God created and ordains is for husband and wife to come together in physical union, one flesh.
In Leviticus 18:22 you would of seen how a man shall not lay with another man. Both Jesus and all of scripture approve of no other sexual union than that between a husband and wife. This is the uncontested historical teaching of Judaism and Christianity, and it is not something that true Christianity is free to adjust with the times. Yes, concubines and multiple wives are found in the Bible, but doesn’t make them “biblical.” In fact, they violate the Genesis narrative Christ points us to.
It is not just mere “traditionalism” that makes sex-distinct marriage the norm for Christians. It is a common grace God has given to all peoples at all times that is rooted in deeper theological reasons. The first chapter of the Jewish and Christian scriptures tells us that humanity is uniquely created to show forth the image of God in the world — to make visible the invisible. God does this not just in generic, androgynous humanity, but through two very similar but distinct types of humans: male and female. They are human universals, not cultural constructs.
When God said that it “is not good that the man be alone” (Genesis 2:18) he wasn’t lamenting that Adam didn’t have a buddy or was just lonely. He was saying that the male could not really know himself as male without a human “other” who equally shared his humanity but was meaningfully distinct right down to every bit of her DNA. The same is true for her in Adam. Taoists understand this in that the Yin cannot be Yin without its corresponding and contrasting Yang. In both Jewish and Christian belief, both male and female become fully human in their correspondence and contrast with one another. This does not happen solely in marriage, but it does happen most profoundly and mysteriously in marriage.
It is a new and culturally peculiar idea that human sexuality is all about intimacy and pleasure, but not necessarily babies. Babies and reproduction matter. And sure, while not every male/female sexual engagement is toward the end of procreation — intimacy and pleasure matter as well — it has been the overwhelming norm and desire in nearly all marital relationships throughout time. That some couples are infertile either by age or incapability does not diminish or challenge this reality. Infertility is the vast exception for male/female couples. It is the fact of same-sex unions, a human cul-de-sac. Heterosexual union reaches into and creates the next generation. To establish a sexual relationship without any interest in or openness to babies is contrary to God’s intention for such relationships.