How is "Epithalamion" a marriage hymn?

"Epithalamion" is a long lyric poem written by Edmund Spenser in celebration of his marriage to Elizabeth Boyle. The word itself means "marriage song," coming from two Greek words that mean "upon" and "bridal chamber." It is easy to follow the poem once one understands that each of the twenty-four stanzas represents an hour of the couple's wedding day. The first few stanzas anticipate the bride awakening, but she is still asleep and dreaming. At the fifth and sixth stanzas, she awakes, and then she dresses for the occasion of the wedding ceremony. Throughout the first half of the poem, Spenser invokes many Greek gods and goddesses to help prepare for the wedding. For example, he calls upon some to help dress "my beautifullest bride." In the eighth stanza, boys run up and down the street hailing Hymen, the god of marriage.
At stanza twelve, Spenser urges the wedding attendants to "bring her up to th'high altar that she may the sacred ceremonies there partake, the which do endlesse matrimony make." At this point, Christian allusions replace the Greek references. In stanza thirteen, she takes her vows and angels sing alleluia. In the following stanza, the bride is brought to her joyous husband's home. Maidens sing and bells toll. Eventually the party ends, and the two lovers are left alone to enjoy their wedding night.
The entire poem is a recounting in flowery language, full of Greek and Christian allusions, of the joyous wedding day. It is indeed a song, or hymn, celebrating marriage. Spenser wrote it as a tribute to his own marriage, but by extension, it can be interpreted as an homage to the tradition of matrimony and the uniting of a man and woman in a lifelong loving relationship.