Such a huge topic! I always like to start at the basics.
Here’s where I always start at the root, for us that believe in Creation, that God imbued in us His likeness and image, and made us human. Here’s the thing, God creates creatures and those creatures can create more creatures without regard, with authority, without any recompense … meaning, we can make babies with whoever we want whenever we want. Imagine the most wonderful gift ever possible, the gift of life, and then God gives that very same power to create to the little “slaves” he just made, us!
How amazing is that responsibility? It’s actually quite magnanimous, but most folks just treat it as a means of self-gratification, using others, pleasures of the flesh, using the gift of procreation for everything but procreation (for the most part)
Men and women are ordered towards union, a life-long union, because human babies don’t fly from the nest at 3 weeks to be gone and never seen again, they stick around for about 18-20 years, and then sometimes they come back at 25! That’s a very long commitment to raise offspring.
Men who are attracted to women, if they have sexual relations with a man, it’s still disordered.
Women who are attracted to men, if they have sexual relations with a woman, it’s still disordered.
So, from the catholic point of view, sex is for a man and woman to have unity and opportunity to procreate.
From the secular, non-religious point of view, homosexual physical relations may have unity, but they are disordered because there is no procreative element. There is no point in being together as they cannot make any offspring. Only option is to adopt. The adopted kid is now being brought into an inherently disordered family, poor kid, kid has to be judged and go through the disorder of a same-sex couple and the kid didn’t ask for it, the kid is there to please the same-sex couple so that they can have their desires filled, regardless of who it affects as long as they are happy, what else matters?