There was a spurious article about one of the famous advice columnists, maybe Abby, who was talking to the rather plain wife of a billionaire. When asked how she kept her husband from straying or looking for “trophy wives”, the wife said “I swallow”.
I really wish I could find the original article but it evades me on Google.
It's really that simple. A wife doesn't have to be beautiful if she's making an effort to keep her husband sexually satisfied. Conversely, no matter how beautiful she is, if she's never in the mood, then hubbie is much more likely to look for a replacement.