I agree with all who said these two have been living in a fantasy world. Their relationship didn't have a natural ending, where it resolved and they knew that they weren't right for each other for various reasons. Instead, they have idealized each other.
It reminds me of Elizabeth Taylor who had always said Mike Todd was the love of her life. No, he was the only one who died before she had a chance to divorce him, like all 7 other marriages.
I kind of wonder if the bride didn't stop the marriage because she didn't want to chance a second time, "the guy who got away." If she had told the groom and
he was the one who stopped the wedding, and she tried again with her childhood love and it didn't work, she would have been a double fool.
I also kind of wonder if the old boyfriend really didn't know she was about to be married? In all those years, he never had contact with
anyone from his past there? Their relationship wasn't a secret. An old friend might have texted him, "Hey, your old GF is getting married next week." He decides to selfishly show up to see what happens. Texting her right before the wedding, giving her one last chance to back out was the same kind of act.
It would be no skin off
his nose if later their relationship didn't work out. He could just leave again. BUT, it was quite a compliment and flattering for
both of them that he DID come back, to proclaim his undying love. More of that fantasy. It goes on record that he tried to win back his old love.

How fairy tale romantic.