The original question included the comment,
It’s not a matter of homosexuality means the population won’t grow or be able to look after us in our old age – as was the case in Biblical times.And elsewhere in the email was the request not to list verses but to explain why this remains the situation millennia later.
I am not certain one can do this staying away from Bible verses per se. Perhaps one can refrain from quoting proof-texts, but defence requires explaining actual texts.
It is not just that we know what God has commanded—though obedience without full understanding is an aspect of faith—he has told us much about why the world is.
Therefore the creation mandates must be considered in any approach to sexuality, we learn much of God's intent for the world. God's creation of humans was the culmination of his all his works of creation; we are made in his image, we are given dominion over the rest of the world, we are told to procreate and fill the earth.
Adam does not find himself a companion amongst the animals, but on meeting his wife:
This at last is bone of my bones/He recognises his need for companionship. God wants relationship, and shows Adam his need. Adam's need for companionship cannot be meet in caring for animals, it needs to be in something—which turns out to be a someone—who shares the image of God. Yet also we see God's intent in creating a woman. God makes her human to share the imago Dei, but makes her female to compliment him as a person. Jesus confirms God's intent by quoting this passage
and flesh of my flesh; (Genesis 2:23)
Jesus answered, “Have you not read that he who created them from the beginning made them male and female, and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’? So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”The two were to be one.
This is primary, and without the added consideration of children. Marriage is for complimentary companionship: humans not animals, two not many, male and female. God has created us like this, and the intent of design is such that intimate relationship will occur in such a situation. Of course the Fall quickly affected this and continues to do so, which I will address at a later stage.
So what of children as a part of family? Filling the earth was an early command (Genesis 1:28) and was repeated (Genesis 9:1). Homosexual relationships are sterile and cannot obey this command. Neither can barren women who are married, nor persons who choose not to marry including those who do so for the sake of the kingdom. Not having children can be an acceptable situation and need not imply sin. Of course sin is defined in terms of intent. It is certainly not the intent of a barren woman to fail to bear children. Her situation is usually a source of grief. And the man or woman serving Christ and staying unmarried has chosen a calling where they should not be becoming (biological) parents. I am not certain that married couples voluntary refraining from having children is a reasonable option (in most situations). So the issue of a homosexual couple entering a sexual relationship needs to be compared to, at minimum, heterosexuals who marry with the permanent intent of not having children. And though the dominion mandate includes populating the earth, something that will happen despite not everyone contributing a child or two, it is not lots of people that is the predominant reason for parenthood. Malachi tells us the why of having children,
The LORD was witness between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant. Did he not make them one, with a portion of the Spirit in their union? And what was the one God seeking? Godly offspring. So guard yourselves in your spirit, and let none of you be faithless to the wife of your youth. “For the man who does not love his wife but divorces her, says the LORD, the God of Israel, covers his garment with violence, says the LORD of hosts. So guard yourselves in your spirit, and do not be faithless.” (Malachi 2:14-16)It is godly children God wants, not lots of people. Failing to have a growing population is not the primary issue, it is not having faithful children.
Caring for parents in their old age is not the primary reason for family, though it is something that is important for children (1 Timothy 5:4). As parental care is not the reason for marriage, a perceived lack of necessity for children in senility is not an argument in favour of homosexual unions.
The difference in the contemporary social situation compared to the ancient Near-East is not an argument allowing homosexual unions. Needing children to care in infirmity was not the reason for marriage; and having children is command for the sake of godly people, a command that cannot intrinsically be obeyed by two people of the same gender.
Part 1. Part 2. Part 3. Part 4.