Everyone who engages in this practice is lacking respect and commitment. No one is compatible with another person, we have to develop compatibility through the work of learning about another person, that is not what we do in bed, it's what we do when we work out dilemmas, care enough to give up our own desires for the sake of another and from choosing to love through our attitudes and actions. It takes the love of Christ to be able to understand this commitment and dedication.
The facts show that those who live together without marriage have a higher rate of divorce than those who don't do it, it comes down to respect. If we do not respect another person enough to commit our lives to them without using them first, then we will not respect the marriage covenant either.
People are not used cars, you don't take them for a test drive, allow a serious bond and then decide later you will not bother making them a permanent part of your being, notice I said being, rather than life.
Romans 7:2-3
"2 For the married woman is bound by law to her husband while he is living; but if her husband dies, she is released from the law concerning the husband. 3 So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man."
God's principles are for the protection of the family and each member in that family.