Uh, hello? Why hasn't anyone mentioned Love? It's the obvious choice for a wedding song. Voyage, also, is a great choice. Who..., of course.
I've always thought Days was a song about unrequited love, about how it can just be as fulfilling as love that is reciprocated. Especially when Ayu sings this part in isolation: "I've known for a long time that you have someone precious/Because it's clear from your smile". It's the only way the next lines about feeling sad make sense and why Ayu would write "I expect nothing special from you/Can I stay loving you forever/Just like this?", so I don't think it would make a good wedding song
M is about the transience of human relationships. Fated is about the struggle to reconcile the idea of fate with the reality of a failed relationship. And Far Away ends with "a summer without you". Guys, read the lyrics first
Dearest, even if it sounds melancholy, is as good a wedding song as Voyage, lyrics-wise. Close to You, Carols, Moments, Jewel, or even a less upbeat version of Angel's Song can work, too.