Because in a sexless marriage, there is still love. A relationship has so many more levels than just sex. Just because there isn't sex any more, doesn't mean the love between the 2 people disappears. No matter how many people i sleep with, no one makes me feel the euphoria i feel just by being wrapped up in my master's arms. Even if i am close with a few of my little bf's (regulars), there is no comparison.
It is a very different thing to share a life with someone, than just to be in the bedroom. And I tip my hat to any man who stands by his wife after menopause even if she goes no sex, especially as this is a major insecutity to just about every woman. Will he leave me for a younger woman when nature takes it's toll and makes sex hard to enjoy, or even painful? They don't go no sex to punish him, often it is because sex became painful or uncomfortable rather than pleasurable.
But there is so mich more to a woman than just sex.
That's like saying a husband with ED should be abandoned by his wife because he can't do anything with it any more. There is so much more between them. A life built together isn't something that you should just be willing to walk away from because of something as trivial as the ability to preform.
Edit: In a good relationship, your partner can also be your best friend. You turn your back on the relationship, you loose your best friend you had for however long you were together. You have guy friends/best friends too, you enjoy your time when you hang out with the boys, you willing to turn your back on them because they won't let you bend them over?