Hi, Bagpiper! I highly recommend having a look at the posts by this user - HisCross.
Our Lord… created man and women for each other, for them to become one in the perfect love which he had made them from, and that they be joined also to him in sacred bond.
Indeed. As the Torah begins, “Male and female He created them”, as well as “that is why a man leaves his family, and clings to his wife, and the two become one flesh”. I think something important is being said in acknowledging that humanity began with men and women.
This joining of them, this love, is the love that produces life.
Indeed. The two become “one flesh”. And how is that? In children. You will never see the child as anything negative in the Bible. The Catechism, at length, details how the conjugal act between a man and a woman is made to unite them spiritually. It also presents near the end of the overview of marriage:
Paragraph 2363 of the CCC:
The conjugal love of man and woman thus stands under the twofold obligation of fidelity and fecundity.
And it goes into detail about both union and procreation. I highly recommend reading the section on the Catechism about the sixth commandment.
Any other sexual relationship is not pure love, as there is no care for bringing life or God.
I would not go this far. True, it is not a pure love. But on the surface, homosexual adoption seems harmless enough. In fact, it seems to be doing a good turn for society. And I’ll say I don’t believe most homosexual couples have any ill will when they raise children. They care; it’s just that there is a lack of depth in the logic behind their care. For ultimately the alleged right to adoption, IVF, and surrogate pregnancy etc has at its root making children a thing to be owned by a parent, and not a person whom God gives as a gift and a responsibility, who is our equal, and who has rights like anyone else. And one of these rights is to be a gift of love between its parents and God, not a manufactured product or carried by a mercenary mother. But most people these days do not see this far in, probably because no one’s ever had to before.
…When these two join together in perfect love, along with Christ who is the center holding them together, then they complete the image of God. By their perfect love, they will bring life just as God brought life.
I believe, if I read “The Theology of the Body for Beginners” correctly, that is basically what Pope St. John Paul II said when he tried to understand God’s original plan for marriage. Pope John Paul II wanted to get to the root simply because, as Our Lord said, “You were allowed divorce because of the hardness of your hearts.** But in the beginning it was not so.**”
So, the Pope went and looked at “the beginning”, which Our Lord so desired for us to emulate. And, to put it briefly, just as Our Lord instituted only men to be His Apostles, though Our Lady was the greatest of all humans, I think there is something important in seeing Jesus turn to the beginning, and seeing that in the beginning there was a man, and a woman, and
these two became one flesh.
If marriage could be between two men, or two women, do you not think it would have entered the minds of men long before our modern age? Do you think Christ would not have brought it up at some point? I realise this is a, really, inadequate response to the OP’s question. But it is food for thought, don’t you think?
We see this in scripture. How can the Church that Christ established go willingly against what Christ commanded?

Believe me, it’s not easy. But you believe Christ founded the Catholic Church. Does Christ lead us astray? Can His Church ever be mislead by God? If on every other issue the Church is correct, why not this one?
And it’s not the only Church that cannot see two men or two women as being really and truly married. The Orthodox, and many Anglican and Lutheran bodies, also see this. I do not know of very many churches that, having compromised on this issue, have not also compromised on priestesses, abortion, contraception, the utter equality of men and women (rather than their complementarity), if not the nature of God as Father and Jesus as Son, among other things. They all link together. The masculine and the feminine are realities of our world that the Catholic Church recognises, that God made and made good, and that the secular world will not acknowledge.
But even if you don’t believe all that, believe, if only because the Church teaches it to be true, that at least the potential to fecundity is necessary to marriage, and this potential even is missing from any group that contains only one sex.
You are young, your understanding of these things needs to grow. The secular world is a powerful force, it is ingrained in every part of the day. …However, if we hold to the truth of what Christ taught, a truth that leads to the source of life, the source of all goodness, then the secular worlds logic crumbles.
Trust in Him who loves you beyond anything or anyone in the world. …It has all to do with the goodness of it, the righteousness of it. The reality of it. The reality that we are doing the Will of the one who holds all things, the Will that is the source of love and life.

Amen.
1 Corinthians 6:9-10
Galatians 5:19-21
…
Amen.
In reference to something HC wrote about people not wanting to follow teaching:
That, or because it seems too hard. " ‘This saying is hard to us; who can follow it?’
… and they left, and followed Him no more. And He asked His disciples, ‘Will you also leave?’ and they said, ‘Lord, to whom would we go?’ "
They are the ones who do not at least have a little faith, a little trust in his promises. … life.
