A marriage proposal should be memorable.
Xavier has been with his girlfriend for years, and they are watching a movie together on their couch.
Xavier wants to marry his girlfriend.
Xavier lets his mind wander during the movie and plots out a unique marriage proposal plan which he goes on to execute.
Xavier broaches the subject of marriage and after confirming his girlfriend is interesting in it he simply that they should indeed do it.
Xavier's girlfriend is moved by the personal thoughts and care that went into the proposal and says yes.
Xavier's girlfriend doesn't even realize at first that Xavier considered that a proposal, and she is somewhat let down when she does find out.