Morals are not explicitly “taught” in the Bible, they are implied. So, not listing them explicitly in the Bible does not mean they aren’t taught in the Bible. A “Biblically moral” worldview presumes and holds truths to be self-evident, among them: morality and the benefit thereof.
Enoch, Noah, Job, and Abraham were called “righteous” men, even before there was any moral code outlined in the Bible. This goes back to the Biblical idea of basic righteousness: using balanced scales—the same standard for oneself as for everyone. This includes morals about marriage.
In Egypt, Pharaoh wanted to respect Abraham’s marriage. There was no Biblical teaching about marriage and what it meant at that time—there never is a clear definition in the Bible of what marriage is. The way to get a “Biblical” definition of marriage is to look at Bible stories that presume marriage as already defined, where the Bible explains respect for that view of marriage. That is a “working definition” of marriage in the Bible, the only kind of definition there is.
All morals work this way in the Bible, even where specific actions are outlined, such as the Ten Commandments, or generic “evil deeds” described by John in his Gospel and in Revelation and other passages where immorality is frowned upon, but not defined.
God expects that we each already, generally know what is right and wrong. There might be some ambiguity in what God expects of us, though the simple rules of humility toward God and fairness toward each other can’t be mistaken. Some “morality” answers are admittedly fuzzy; some (never all) of those fuzzy answers God allows discretion of each person’s conscience.
Even with the ambiguity on some Biblical morals, nowhere does the Bible allow us to rewrite God’s morals nor does the Bible ever even suggest that morals are purely at the discretion of Mankind to create and alter.
But, never quibble over morals. Run with the people you agree with; respect those you disagree with. We must each answer to God for our own lives, never for other people’s. But, we will certainly answer for how we understand and practice morals, whether clearly presumed, defined, or fuzzy.
Genesis 12:10-20, John 3:19, Romans 2:1-16, 1 Corinthians 4:4; 10:29, Revelation 9:21; 16:11; 21:8; 22:14-15