It's not purely that, though.
There are definitely more unattractive/average male actors than female ones, but the important difference is that women care. It's the fact that we do compare ourselves to others. We are socialized to worry excessively about our appearance - everything from our makeup and clothes to our weight. It's simply an expectation that pervades our society.
Think about it:
As children, girls (more than boys) are read fairy tales where the good, kind figure is always the beautiful princess; meanwhile, any character with an imperfection is the villain. Girls are also presented with barbie dolls, and spend a disproportionate amount of time dressing them to make them "look pretty". When they themselves get all dressed up for special occasions, they inevitably get praised by their parents' friends for "looking so cute in that dress" (leading girls to associate appearance with attention).
There are a million ways that women are taught the importance of appearance. Beautiful celebrities are only another chapter to what we've already learned.