The purpose and intent of Marriage and Sex

Adam couldn't receive, much less fulfill, the command to be fruitful and multiply prior to the creation of Eve.  All the rest of creation could procreate but Adam could not do so on his own.  Creation requires a way to procreate in order for  a life form to persevere and so it is in Eve that God's creation is perfected and the future of the image bearers secure.  The relationship between humans, however, is not the same as the other animals, it is deeper in every way and intended to last.

For the two to become one flesh expresses the relationship of the Godhead, God in three persons who are One.  (See the Creed of Athanasius)  In theology we use the term perichoresis when we speak of the relationships of the Trinity.

Perichoresis is a Greek term used to describe the triune relationship between each person of the Godhead. It can be defined as co-indwelling, co-inhering, and mutual interpenetration. Alister McGrath writes that it "allows the individuality of the persons to be maintained, while insisting that each person shares in the life of the other two. An image often used to express this idea is that of a 'community of being,' in which each person, while maintaining its distinctive identity, penetrates the others and is penetrated by them." (Source - Theopedia)

The two image bearers are to have the same relationship as the persons of the Trinity.  In the high priestly prayer of John 17, Jesus prays for his followers that they would be one as He and the Father are one.  Marriage is the ultimate fulfillment of the unity of persons and sex becomes sacrament rather than simply the fulfillment of lust. 

Within marriage lust is not wrong, but if sex within marriage is nothing more than satisfaction of lust, it loses the deeper significance we should see in it.  We should take seriously the reality that in sex we become truly one flesh in a unique and God-ordered way, it is the highest expression of love and our appreciation for the created order that allows sex to be both for pleasure and reproduction.

Marriage is intended for many reasons, not the least of which is reproduction of new image bearers that God might never lack a representative on earth.  That shouldn't be reduced to a slogan that belittles other forms of relationships, it should, rather, emphasize the sacred in sexual reproduction.  Our union has a higher purpose than that of other animals and we have cheapened it through the sexualization of all things.  The first place to reclaim marriage as a Christian concept is at this point, reclaiming the primacy of marriage as the domain within which sex is to be confined, like the old illustration of the fire which is good in the home if confined to the fireplace but bad if it is found elsewhere.