There are no guarantees!
Case in point Cindy Crawford & Richard Gere, Brad Pitt & Jennifer Aniston. Both couples were considered to be made up of "beautiful people" and those marriages ended up in divorce.
Beauty is in the eye of the beholder.
Very rarely do relationships end because of anyone's looks unless there has been a major change in their appearance after they got together. (packed on a lot of weight..etc)
Most relationships end over bad behavior, attitudes, or when couples are not on the same page/wanting the same things.