A Roman Catholic wedding is a long, complicated event.

Besides all the cultural wedding customs and various traditions (i.e. something old, something new, the rehearsal dinner, which family pays for what, etc.), there is this whole other set of religious protocol to follow. To wit, you will usually have not only a short wedding ceremony, but also a full mass. The wedding sacrament takes place between the Liturgy of the Word (i.e. readings, gospel and homily) and the Liturgy of the Eucharist (i.e. communion). When you put all the ceremonies together, you may be in the church for up to two hours. Be prepared.

Most Catholics will want to have a wedding with a mass; a couple can request only a simple ceremony, and a priest can perform one, but there is a bit of a negative connotation to this. Plain marriage ceremonies are generally given to those couples who have some sort of problem with the church, or with whom the church has some kind of problem. If a partner is non-Christian, for example, that couple may not be allowed a full mass. If two unconfirmed Catholics want to get married, they will almost certainly not get a full mass, since they are not full members of the church. In short, a short wedding is a punishment: you are not allowed the full benefits of the church; you do not receive as many blessings (etc.) as those who are having a full mass.

This seems to be a remnant of the Catholicism of the middle ages; people used to buy masses along with indulgences in order to purge their own sins or the sins of their dead relatives. If you have a full mass with a wedding, then you are seen as being blessed and purified much more than a common ceremony. Your marriage will be recognized not just by the priest, as representative of heaven, but by heaven itself and the trinity thereof. And you definitely want the approval of heaven on your wedding. If you aren't recognized by god, is your marriage really valid? Is your family valid? Are your children bastards, and will their baptisms be valid?

Yeah. So it's some heavy stuff. I don't know how many people take this into consideration--probably many of them simply want to get married in a full mass because the Catholic church is their church, their family, their community, and a part of their life. But if you dig deeper, this is what you will find.

sources: my head, http://www.ultimatewedding.com/articles/get.php?action=getarticle&articleid=638, http://www.newadvent.org/cathen/10005a.htm