OP..
If you take "most" out of the equation, then you will be correct. Saying most implies that there is some sore of empirical evidence to support that claim...
Nevertheless, being with an extremely attractive person could cause problems in a relationship with an extremely attractive person.
This is why it may be in one's best interest to deal with someone on the same attraction level as themselves.
Also, it doesn't hurt for a person to be secure within oneself when dealing with attractive people because I believe one of the reasons why there may be issues is due to someone having insecurities which leads to jealousy, codependence, control issues, etc...
Lastly, there may be other factors that cone into play such as infidelity, narcissism, unresolved emotional trauma, immaturity, etc. Not to mention, it should be understood that looks do change as people get older, so some attractive people do "fall off" sort of speak, but usually the personality never changes...
Something to

about...