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When we assume that the main purpose of a human is to reproduce, then the optimal sexual orientation for a man is to perceive women as attractive and for a woman to perceive men as attractive. This optimal orientation avoids individuals wasting energy in sexual efforts towards a partner of same sex, which would be useless.
If you continue this thought with a politically right-wing mind, you come to the conclusion that a healthy civilization should repress homosexuality as a suboptimal development that might even harm your civilization by subverting divine laws.
But things get more complex when you consider that the meaning of life is more than having children, and that increasing our population (the number of individual humans living on earth) is not a primary concern of our species.
The Catholic Church has no problem with marrying a couple who know that they won’t have any biological children together, e.g. for medical reasons. Such a couple may have other kinds of “children”: they can adopt orphaned children, they can work together on spiritual “children” (a project or a vision) or invest into material “children” (a house or a car).
This shows that a marriage makes sense also without sex and without biological children.
So I see no problem to allow for same-sex marriage as well. I believe that the Church should recognize same-sex marriage as a variant that deserves the same benediction as any other marriage.
There are homosexual people who agree to form a couple and even to adopt and raise children in a family-like environment. There is nothing wrong with this. It is great and deserves our support and benediction. Because children having a homosexual couple as parents are still far better off than children who grow up with only one parent. Homosexual people should have a right to form a civil union, to marry and to adopt and raise children.
There is a theory that homosexuality might be “nature’s population control so that we don’t overpopulate.” Richard Dawkins responded to this idea on Darwin Day 2015. He explains that evolution “doesn’t work like this” because it works on the level of the genes of an individual. He concludes with a –for a Britain– surprisingly clear verdict: “So I am afraid that it couldn’t be true that homosexuality is an adaptation to keep the population size down.””
See also About family.
“When it comes to homosexuality, many Christians are being stupid and unfair to both Pope Francis and Saint Paul.” – Homosexuality, Francis & the Bible, Patheos, October 2020.
“Homophobia forces many Christians to disproportionately apply ethical gravity to two obscure Greek terms while neglecting other biblical vices.” – Ethical Consistency & Homosexuality, Patheos, November 2020.