Marriage is designed by God. God made Adam and Eve. Not Adam and Steve. That is the only fact you need to know. If it had not been that way, we would not be having this discussion right now. Because there wouldn't have been pro-creation. Two men cannot have a child. Two women cannot have a child. The human race would die out.
When did Adam and Eve marry? Was their marriage of the common law variety?
Read the story.
Personally, I like the types of stories that can be verified myself. That's not to say that the bible isn't a very interesting book. I have read books and seen a film that have brought up points in it that I found to be quite intriguing. What began to get my interest was when I read a book called "The Da Vinci Code". After seeing the movie of the same name, I was even more interested. One day, while checking out a bookstore, I found one of the books it was based on, "Holy Blood, Holy Grail"- unlike The Da Vinci Code, Holy Blood, Holy Grail was not a book of fiction, but an investigative work by 3 authors. I've since read other books discussing the bible. What they all have in common is that they don't see the bible as a book written by God, but instead a book written by men. The difference is a large one.
I confess I have not read much of the Bible, but in my continuing readings I have not found any excerpt declaring the words were written by God, generally omitting singular authorship. Many references are made to scribes, and also to the various forms words take (spoken, written, thought of, numbered, codified, categorized, and all the more complex). The translated versions generally have the academic team who worked on producing that specific version in the introduction, either informing further on what version it was translated from or at least from what language, so that you can continue your study unto established knowledge (in the final form of the book's total content, whatever version or language it may be in).
Compare, for example, the first passage of the whole collection with the first passage of the book of John (after 30 and more books within the Bible):
Genesis 1:1 said:
In the beginning God created the heaven and the earth.
and
John 1:1 said:
In the beginning was the Word, and the Word was with God, and the Word was God.
The work is colossal, but there is nothing lacking in it, and it comes to be especially complex because it compresses so much of its full range and procedures. Only about 1000 pages total, but then thousands upon thousands of separate works referring to it, from books, paintings and songs to complete architectural buildings intricately decorated with all kinds of materials and made to be comfortable and welcoming to any interested party.
There is nothing about the Biblical story that cannot be verified by you. It's not simply interesting, it's educational.