A relationship between a concubine and a man is that of fornication without commitment. They aren't in love with each other, or they would commit. They are just using this person to supply their physical needs. A fornicating relationship is not a relationship of love, it is lust.
A man is suppose to be willing to lay down his life for his wife. He should be willing to work, to provide, to protect; his life is all about supplying her needs. That is love. Love respects someone enough to wait for marriage. Love will commit to that person before they fulfill their fleshly desires. When a relashionship begins with fornication, it plants a seed of alienation and drains the love out of the relationship, and brings the curse of God upon both the man and the woman. It's not something that you want to spend the rest of your life paying the consequences for!
True love obeys God's commandments and doesn't ensnare someone into a shameful situation that expects you to disobey God's will for your life to "make the situation right". There's a big difference between spending your life with someone and sacrificing your life for someone.
