Sarah asked me in my last post's comment thread if I think marriage is a sacrament. I'm on the fence about this one. The reformers in general thought that there were only two sacraments: baptism and the Lord's supper. This is in contrast to Rome, which has 7 sacraments (adding marriage and 4 which I can't remember). The Reformer's reasoning on this was basically that marriage isn't presented as a sacrament in the Bible. God tells us where and how he gives grace, not the other way around.
I originally thought that a good argument against marriage as a sacrament was that it wasn't instituted by Christ. But then I remembered that Jesus attended a wedding at Cana. That could perhaps be implicit institution. And then I remembered that Christ is married to the church. So that, at least, is a poor argument.
If marriage is a sacrament, it is at least very different from the other two, because it is not commanded... in fact Paul advised the Corinthians not to marry. I'm a little afraid that if marriage is allowed as a sacrament, then pretty soon we'll need a sacrament for single people, and not long after that we'll be back up to seven.
Personally I think the idea of marriage as a sacrament is really cool. It certainly has the sign/thing signified structure of a sacrament: the union of husband and wife signifies the union of Christ and the church. But being really cool isn't a sufficient condition for being a sacrament. In fact, the tendency to teach stuff that is really cool as doctrine is one of my biggest criticism of Catholicism (especially late medieval Catholicism). On the other hand, maybe a good Biblical case could be made (from Eph. 5, etc). The jury is still out on this one for me.