The processional has already taken place, so why is there a hymn here?  At most parishes, this hymn is omitted.  But at St. Martin of Tours, the pastor prefers to have a hymn here while the couple processes from the aisle of the church to their seats.  Many feel that having a hymn at this point in the liturgy serves to encourage the congregation to participate in spoken and sung prayer for the remainder of the wedding.  Any song of praise or gathering would be appropriate here; these are a few suggestions.

Here Us Now, Our God and Father
Joyful, Joyful, We Adore You
All Are Welcome
Gather Us In