Raptor wants to share in the joy too. Either that or eat you.
What better time to pop the question than after saving Kanto from Giovanni's reign of terror, freeing the gyms from Team Rocket, and rescuing all the kidnapped victims, including the rival's wife? That is, of course, you unlock the secret ending.
Hopefully you'll let Team Rocket blast off to the wedding. I mean, that way at least it won't be boring!