The key element to remember about the Catholic understanding of marriage is that it is a public act of the church which recognizes the lifelong and exclusive commitment of the bride and groom to each other. The bride and groom may say “ I  do” to the wedding vows, but the presence of the church community is meant to support the couple throughout their married life together. All those present are presumed to be saying to the couple “ We do.”  We do witness, confirm, and support your marriage.
The popular notion that a wedding is primarily the business of the bride and groom is romantic, but not true in the sacramental sense. The church, and all the people of God who witness the marriage, have a stake in the sacrament of marriage. It makes a difference to the community of believers and to society that marriages are freely entered and strong. As Pope John Paul II said, “The future of humanity passes by way of the family.” (Familiaris Consortio, #86)
What’s all this got to do with having a wedding in God’s beautiful outdoors?
Since sacraments belong to the entire church—not just the bride and groom—they are normally celebrated in the place that the church gathers. This unites the couple with the universal church throughout the ages and puts the ceremony in the common gathering place where other sacred celebrations occur.
Although as Christians we believe that God is everywhere, we also have set aside special places for community worship—church buildings. It makes sense that baptized Christians would celebrate the vocational sacrament of marriage in the building where the community usually worships and which is dedicated to such special sacred commitments.
Are there any exceptions?
Yes, no and rarely.
If a Catholic is marrying another Catholic or baptized Christian, the wedding vows should be exchanged in a church building. If both partners are Catholic, of course this would be in the Catholic Church building. If one partner is baptized but not Catholic, the ceremony could take place in the church building of either partner.
If a Catholic is marrying a person of another faith—a Jew, Muslim, Hindu, atheist, etc.—a special “dispensation from sacred space” can be requested from the local bishop. This is sometimes appropriate in respect for the faith of the non-baptized partner.
For example, if a Jewish person is marrying a Catholic, it is not permissible for Jews to celebrate the wedding in their own synagogue. However, it is permissible to use an appropriate neutral location.
In situations such as this, the criteria is that it be a place of dignity but not outdoors. Often reception halls serve this double duty as both the place of the ceremony and the place of the reception.
So yes, there can be a dispensation for a wedding outside a church building for good cause, but it is rare. A dispensation for a wedding to be held outdoors is even rarer.
If you have further questions, please call the Diocese of Austin, Marriage & Family Life department.