Why does the church retain some aspects of the old law, for example the prohibition on homosexual behaviour, but not others, for example the kosher laws? How can this be justified?
When we speak of "The Bible" we are in many ways using a misleading phrase. The Scriptures are not one book, like "Great Expectations" or "War and Peace", but a library of texts, some pointing forward to Jesus, the liviing Word of God (the Old Testament), and others reflecting on that Word once it has been experienced (the New Testament).
So reading the Bible calls for different ways of understanding texts, and not expecting every passage to be the same. There are poems, songs, stories, moral advice, letters in the scriptures. And the commandments given in the Law are just as diverse, needing to be read in different ways.
The Church distinguishes in the Old Testament three main types of commandment. There are the moral precepts, which are part of how to live a full human life. All human beings can learn and engage with these teachings, whether it is about not killing, honouring your parents, or the nature of sexuality. These are usually called the moral precepts.
There are also commandments about how to worship, and how to live out justice in the particular circumstances that the people of Israel will experience, living in a particular time and place. These commandments often called the ceremonial and the judicial precepts point us forward to Christ, and although they contain much that can edify and instruct us, they do not apply to us in the same way.
By renewing the people of God, and calling all nations to enter the Church through baptism, the precepts applying to Israel as one nation are taken up and fulfilled in Jesus. But the teachings which reflect on what it means to be human apply now, simply because our human nature remains, even though it now has access in Jesus to the grace which can heal and restore that nature as belonging to God - what it was always supposed to be.
What the Church teaches about human life is for everyone to engage in. Often we find the teaching of the Gospel difficult to follow, but truth spoken by God in Jesus is not a truth to break us or destory us, but to lead us to our true and lasting joy - friendship with the Trinity.