The memetics of marriage
1. How it applies to the Christian memeplex.
Marriage basically provides added incentive for people to remain together. Therefore marriage supports social stability. Tthis is a boon for statist governments as well, who provide their own numerous and powerful incentives to perpetuate marriage - the concept of hierarchical life is favourable to the implantation of hierarchical obedience to authority figures.
It also ensures that children will grow up supported by two adults and, presumably, a proper religious education, as the pressure of the responsibility of marriage and children will keep the parents together even if they wish to separate in many cases. And as I already pointed out, children are paramount in religious growth.
Control over the process of marriage ensures that Christian sects can impose their own model of the family on a given society, in this case one man and one woman, with the man being the dominant partner.
Also, in the specific case of priests, it seems clear from history that celibacy was imposed in order to stop priests from bequeathing church property to their children. So here again we see the memetic goal.
2a. Biblical support.
In the Old Testament, marriage is considered so important that people are forced into it, for example in Exodus 22:16 and Deuteronomy 22:28-29 :
If a man happens to meet a virgin who is not pledged to be married and rapes her and they are discovered, he shall pay the girl's father fifty shekels of silver. He must marry the girl, for he has violated her. He can never divorce her as long as he lives.
Jesus is likewise adamant on the issue :
But I tell you that anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery.
Paul is considerably more lukewarm, but this may have to do with his possible repressed homosexuality.
2b. Lack of rational support.
While there is, of course, nothing wrong with commitment, there are also few reasons to establish it as a social structure. There are, however plenty of reasons against it, the first being that marriage is an attractive institution for collectivist belief system, and that they will seek to promote and exploit it as much as possible. Individualism, inside or outside of marriage, gets left behind, and optimal decisions are diverted for social ends.
If marriage was divested of its political power, then it would be a simple ritual, and there'd be no need to make a big fuss over it either way.
Marriage is both heavily promoted in the Bible as well as in the modern Christian worldview, but only within the Christian model and under Christian power. In this way child births, social stability and religious child-rearing are maximized. This goes counter to modern values, which strongly select for sexual freedom, against discrimination of any kind, and against the imposition of specific models. This tension only reinforces the Christian hostility against "the world", in this case non-Christian relationships and sexuality.