The proverb is false. Beauty is not in the eye of the beholder. When people are asked to judge physical attractiveness, there is always a high level of consensus, showing that beauty is something real which people can independently recognise.
Sex and sexual orientation don’t make any difference either: in experiments, everyone ranks the subjects in more or less the same order quite regardless. It’s not dependent on race. Even though features differ from one part of the world to another, we are all still consistent in rating “more beautiful” or “less beautiful” faces or bodies.
It has also been noticed that couples tend to rank at about the same level of attractiveness as each other. (Rich, ugly, old men with young, beautiful women is a minor statistical blip.) It seems that humans instinctively know their own place in the pecking order of attractiveness and tend to “settle” for the first partner who meets or exceeds that.
In evolutionary terms, having a “realistic” expectation like that is a better strategy than constantly holding out for a perfect partner. Those kinds of genes would tend to become extinct. But nature has another trick that makes us think that we have actually found the perfect partner. We fall in love.