This is a really great question. And believe me, I'm not a Biblical scholar on this subject at all, but here are my initial thoughts as they come to me.
First, "marriage" according to who? A part of me rebels against the idea of the secular world determining my marriage. When I was married, I took my "vows" at the "Little Chappel of something or other" in Las Vegas. How repulsive. It was dirty, dingy . . . just a disappointment. Meeting with the "minister" seemed weird to me; he tried to make it seem like a reverent thing we were doing, but it was cheesy.
My second thought is this . . . if "you" are currently sleeping with someone, that's your last! Whether you're married or not, you are not authorized to be with anyone ever again. Paul taught several things regarding marriage, but here are what I think are the two main points he made:
1) Don't divorce, or separate, unless adultery occurs.
2) If you divorce, or if you're single, remain in the position that you're in. Do not change!
Paul gives an allowance that if a person cannot control themselves, then by all means, remarry. But, we are not meant to enter into sexual relationships so that when things go even slightly sour, we just jump right into another. I did that and it is not good . . . at . . . all. I got used to the idea of finding someone new virtually whenever I wanted. As soon as one relationship ended, I was back on the Internet Dating sites that same day. Same day! I became desensitized to relationships compared to when I was young (and still a virgin). When a relationship would end, at that point in my life, I was devastated. I would even go a couple years between relationships because of my broken heart, but once I got that taste for the thrill of "a new woman" and her physical flesh, it became a sport for me. In the end, I was nothing less than a user of women for my own personal glory. Later in the week, I'd revel over my conquests and brag about my proclivities.
There are over 6,000 varieties of apples in this world and God allows us to try them all, but this is not His mindset with humanity. God wants to see that we can be nearly as dedicated to our spouse as we are to Him. If we aren't dedicated to our spouse, it's nearly impossible that we will be dedicated to Him. If we can Endure our marriage, then perhaps we can Endure a Holy life with Him.
This life is not about sex; it is about Endurance and Devotion. Who will Endure? Who will commit themselves Wholeheartedly?