First, I tried for at least 15 years to pray the gay away. I'm not one to do anything half-assed either, so I war full force in my faith and trust in the deity I was brought up to believe existed.
Didn't happen. I left the church because the misogyny and the homophobia was too much for me to handle. I embraced my bisexuality as natural, found peace and happiness, and 20 years after leaving the church, I'd rather stick my hand in a blender than go back. I'm a much much happier woman not being enmeshed in dehumanization.
It's already around me culturally, though, with the culture still sexist and homophobic. It's changing, though it's slow. We have ways to go.
Second, all this "purpose of sex" talk is what has caused great harm to humans around the world. Resulting in spousal rape, sexual assault, genital mutilation, child brides, and second class citizenship for women and girls around the world. People may see an institution as more valuable than the lives and safety of women and girls around the world, but I see the reverse as having more importance.
And this is all with the assumption of everybody being naturally heterosexual on the planet. That's false, and then building on that idea that same sex marriage is a perversion of what a followers of a deity claim to have intended shows how fragile the argument is when we see and witness countless non-heterosexuals engage in meaningful, loving, respectful, monogamous, long term relationships as much as heterosexuals.