But since I think humor is often a useful spice when exploring something new, add some conflict to the marriage. The poet can be struggling to be serious or using backhanded praise to release their ire into the poem with the hope the couple never sees it. (But what if the couple does? The poem could be part of a fiction piece about what happens afterwards.)
It can be a real couple, two of your original characters, fan fiction or cross overs.
If you need some ideas, the couple could be your (not so?) best friend who is marrying the one you were infatuated with.
They could be two people -- they of course don't need to be human or of the same species -- who are so different from each other that unexpected visions of how their oil and water will interact pop frequently into your head.
It could be one who has betrayed your shared ideals to "go to the dark side" and marry (one of) the "enemy".
It could be two people who are both not who they pretend to be who each thinks they're using the other for their own gain.
It could be two clueless people who are now responsible for each other.
It can be marriage number 8 and 6 respectively for the two.
For a different kind of tension, it can be an arranged marriage for two who have met for the first time on their wedding day and instantly clashed, with the poet having known both well since they were children (who, perhaps, realizes the two are made for each other.)
Or, of course, whatever inspires you.
If you're not used to writing poetry, first try brainstorming what the poet would comment on. Don't be concerned about poetic phrases, just jot down ideas.
Most epithalamiums (or epithalamia) share certain features:
- tell something about the wedding day
- praise the bride and groom
- sometimes tell about the bride and groom's past
- give blessings for the marriage and good wishes for the future
Notes on the poetic form from The Teachers and Writers Handbook of Poetic Forms.