I like this answer.
I am looking for answers that would be brought into discussions with non-muslims.
The general population here in the west do not consider marriage. I know of people who have not gotten married but have been together for a decade and have conceived a child from such a relationship.
I would argue that the psychology of marriage vs premarital relationships is that the former would psychologically program both parties to be more committed to the relationship, and commitment as a moral and ethical value would be ingrained in the person so that it would work for their benefit in other areas of life.
Others would argue that they have been in a relationship for so long without any problems with commitment or infidelity. I personally would answer the question in two ways: Firstly, Islam tries to protect the majority. The majority of people have a problem with infidelity and the general population isn't as committed as these rare or low percentages of people that may be able to make it work without being married. Secondly, to me, it seems hypocritical to suggest that one is committed yet does not got married straight away. Marriage is of the souls, not the bodies.Like monad suggested, it becomes a contract between the two parties, an area that premarital relationships lack.
Could people build on these points please?