Why can’t a Catholic couple get married outdoors? We want to get married in the outdoors where God made everything and we can be surrounded by his creation.

Customs differ widely between cultures regarding the place of celebration for marriage. In Western culture, the traditional place of celebrating a marriage is in a church in order to symbolize the sacred nature of the act being performed. Thus the Code of Canon Law specifies that:

§1. Marriage between Catholics or between a Catholic and a baptized non-Catholic party is to be celebrated in a parish church; with the permission of the local ordinary or the pastor, it can be celebrated in another church or oratory.

§2. The local ordinary can permit marriage to be celebrated in some other suitable place.

§3. Marriage between a Catholic party and a non-baptized party can be celebrated in a church or in some other suitable place. (CIC 1118)

Note that a distinction is drawn between the marriages of baptized individuals (§§ 1-2) and marriage between a baptized and non-baptized party (§3). This suggests that the stronger requirement for celebration in a church in the former case is intended to emphasize the sacramental character of the marriage, since the former marriages are sacramental while the latter is not.

It is also generally the case that a Catholic marriage is celebrated in the context of a Mass, and the Mass must normally be celebrated in a sacred (i.e., consecrated) place (CIC 932 §1). Out-of-doors doesn’t count.