It can happen in anything, movies, games, manga etc. I think he didnt want it to be linear and have a bog standard ending so he wanted it to be special but then one idea then another and before you know it you cant undo what youve just did, graduates, marriage, cruise ship then George of the Jungle.
Sometimes though keeping things simple can be better. Personally Id have had cruise ship, return home then have a threesome and probably end it with them both pregnant and/or alternatively he sets his sights on another girl