If you put together all the stuff about sex in the Bible, it can be condensed into 2 rules: 1-you must be married to your partner, and 2-your partner must be of the opposite sex.
The Bible has no rules about what marriage is other than a commitment to be together for life. There is nothing about a ceremony, a license, or registration by a state. In the earliest mention of a wedding in the Bible, it was some kind of family celebration followed by consummation. All the rest came later as a necessity to prove that you're married for the purposes of inheritance. Without secular laws on marriage, it can get pretty nasty when a guy dies and nobody knows who gets what. To the ancient Jews, lineage was very important. Keeping track of family lines wasn't scriptural but they believed that it must be recorded. Jesus went so far as to say that keeping long lists of family lines wasn't to be done.
