The outdoors can provide a very pretty setting for a wedding, as well as a great place to take pictures.  Many people have romantic notions about an outdoor wedding. Television and movies make it seem too wonderful.  But besides the usual problems with poor sound, heat, bugs and everything else that goes along with being outdoors, it just is not the proper place to celebrate a holy Sacrament.  It would be like having a wedding reception at a fast food place or at a construction site for a new home.  Those places are important and necessary, but they are not the place to hold a reception.  It is the same with the Sacraments of the Church.

We celebrate the Sacraments where the Church Community assembles in a sacred place before God and in community as His family.  The Sacraments are actions of the whole community of faith, not just of the individual or individuals receiving them.  Thus, the whole Catholic Christian Community recognizes and acknowledges the marriage, when it is celebrated.  The building where the Church assembles is a room of memories where our journeys of faith find their collective roots.  It is the place where we ask the community for their prayers as we begin a new life together in holy marriage.  If we live and practice our faith, and if we realize the importance of the Church Community, the only proper place for the exchange of vows is in the church, "before the Church's minister and this community".  Here we indeed receive a special Sacrament so that we are enriched and strengthened, not just for a wedding, but to live the Sacrament of Marriage.
"The goal of one's life should not be to find joy in marriage, but to bring more love and truth into the world.  We marry to assist each other in this task!"   (Leo Tolstoy).