Did it happen after Yuna gives her good speech about Spira? how do we like know this? Well at the beggining of X-2 Lulu says she has a baby but the gulwings already know. We know there was alot of romance between Wakka and Lulu but how exactly did they get married?
Anyway, the reason I'm posting this is because in Shinra's Dossier (I think that's what it's called. It's his guide of characters in the game) in FFX-2 if you go to Lulu and choose her to read about it, it tells how she was a guardian on Yuna's pilgrimage, a black mage and now she lives peacefully in Besaid with her husband, Wakka. So I think the game's confirmed that the two are married definitely.
.