This is true only because we are looking in retrospect after the coming of Christ: that many, indeed, turned out merely to be out of God's temporary provision to prepare them for the coming of Christ. This includes giving them a sense of sin, holiness, and above all the inadequacy of the Law of Moses, as St. Paul wrote.

But with Christ's coming, there is no longer excuse for us that some teachings are provisionary or culturally conditioned. Even the kosher law was abolished not out 'appropriateness with the times'. In 1st century Judaism, there is no more inappropriate teaching than abolishing the kosher law. Rather, it is Christ that replaces the whole law, now seen as merely a preparation for his coming.

When Christ declared that man is wedded to woman, therefore, he did not merely teach out of cultural context. He referred to 'the beginning', the original intent of God. This is what the Church understood by marriage as taught by Christ. So it is not merely out of natural law, as some suggested here, that the Church's teaching on marriage comes from. Natural law can be a source of that teaching; but Christ's own teaching is the solid bed of foundation.

Now, you can actually come up with thousands of arguments of how Christ did not actually teach it in the bible. But to do that means you misunderstand the relationship between the Church and the bible. The Church understands all the teachings of Christ with or without reference to the bible. The bible is a condensation of apostolic faith, but not its reduced, exhaustive form. Christ's teaching permeates the life of the Church. The Church is always conscious of what he teaches, because he is the continuation of Christ's body on earth. This is where the Church's conviction comes from.