The idea that males and females of approximately equal physical attractiveness are likely to select each other as partners refers to the Matching hypothesis.
In the field of social psychology, the idea that people are more likely to form successful relationships with and express liking for people whole level of physical attractiveness roughly equals their own.
A good example is. A man of average attractiveness goes on one date with an extremely attractive person, and then another with an average-looking person. He finds he likes the average-looking date more.