I've never understood this either.
Breast alone do not grab attention from most males. If they did, porn would not consist of showing the entire upper body, or more. They would just take pictures of perfect breast, and all males would be happy.
Just as I'm sure the majority of straight or bi females are attracted to the entire torso of a male.
It is the entire package that gets the attention of both males and females. You can't slap on a pair of perfect breast on a 100 year old women and expect every straight or bi man to start drooling.
I think if you are going to force people to cover their breast because a very low percent of males get turn on by breast alone, then you will have to cover feet, lips, hands, ears and any other fetish that some men have.
Our culture actions doesn't seem to have a problem showing female breast.
eg. link link link link
Clearly in our culture women are encourage to show their breast. It is the female nipples that seem to be taboo.
The double standard will probably fade in the next few generations, just as many other things that were taboo years ago...