Here is my experience: I am a fit gym rat. Wife is not....very, very pretty and 15 years younger. Also 20 lbs overweight.
We had no sex life. I even made an excel spread sheet documenting her constant denial. However, before I left her I explained why and how we could save our marriage. We have an 11 year old son together.
First, I cleaned up my side of the street. I was fat myself, but now stay close to contest shape. I explained to my wife that she deserved someone she was attracted to and we should separate if she did not want to be with me physically. She decided she wanted to stay together and our sex life turned around.
It is not great, but I can live with it. She is a sweet, kind and giving woman, makes a shit ton of money and is generous. She does very little nagging and I do not allow us to argue. She is an excellent cook and mother.
She is still 20 lbs overweight and allergic to the gym, but also has a 10 face. I am old and ugly.
All in all I stay with her. Sure, I wish I had a fit bikini competitor for a wife, but you can't have everything. Taken as a whole, my wife simply has too many good qualities for me to leave her (which I would do BEFORE cheating).
You have to look at a marriage in its totality, weigh all the good points against the bad.