The wording of a beautiful wedding message should match the desired tone: elegant, beautiful or formal. As with any writing, it is important to be able to communicate a message concisely, but in the elevated emotional context of a wedding, longer sentences and poetic language and imagery have a place.
The language of a beautiful wedding message should revolve around feelings and images. It should be centered around praise, congratulations and hopeful expectations for the future. In particular, images of growth, unity and harmony are fitting for a wedding message. Common metaphors include flowers (the idea of blooming) and symbols of domestic partnership, such as crossing a threshold together or making a "recipe" for a successful marriage. One other way to frame the language of a wedding message is to talk about the unity of the couple's love as a force able to overcome challenges, as in "the storms of life" or "the trials and tribulations of marriage."
There is a wealth of resources on language suited to love and marriage. Poets and authors have written extensively and profoundly on the subject. If poetic phrasing and emotional language don't come naturally, consider using a quote or excerpt of poetry and then explain how that writing relates to the relationship between the married couple.