Doing my best to ignore the assertion that I'm not using a moral compass but just having an opinion! I have an opinion that is based on what God tells us about himself and about the way he made us. What God wants and intends is my moral compass, as far as I can manage it.
Right. Here goes. Why are men and women to have different roles? Why doesn't God want people to have homosexual sex? Both questions involve looking at how God intended men and women to be.
God made men and women differently, with different roles. I could just say 'he made them different because he wanted to' but actually there's a deeper reason. I think the story of Adam and Eve is an analogy, but it shows us that God made men and women to complement each other. They have a relationship of differences that together makes a whole. Sex makes them 'one flesh' , not as two identical parts fitting together but two complementary parts.
That would be why God would not approve of homosexual sex - that complementarity is missing. In marriage men and women 'fill in each other's gaps', working together to be more effective than two separate people.
There's also the issue of children - while children are not necessary to a marriage, it is via heterosexual sex that children are conceived (naturally). One of God's instructions to his people was to multiply - something that involves a man and a woman, not two men or two women. Of course it's possible for people to have children via artificial means, but it's still something that involves eggs and sperm, women and men.
Another aspect to this is that God explicitly compares the relationship of Christ and the church (or God and his people, if you prefer) to a marriage. Christ is the bridegroom, the church is his bride. They are not the same - they have a particular relationship where their roles are different. Christ is head of the church and gave himself for it. Similarly, husbands are supposed to love their wives sacrificially!
So if, say, two men were to marry, how can their marriage be a picture of Christ and the church? Two men would each have the same role, and that isn't the picture that God has chosen for marriage.
So that explains why God says men and women are different and marriage is for one woman and one man. Whether you agree is of course another matter.