Why do fashion models have boyish bodies (very thin and flat chested)? Can it be because of majority in that field is straight women and gay men?
What is more important in fashion modeling, especially runway, is the measurements of the model. The average chest measurements are 36-40. Now in terms of bodybuilding, the men in the picture are still flat chested even though they are toned.
However many male model never had a desire to be cut nor toned and were discovered into modeling or late bloomers. Some of them could have been ugly ducklings, so to speak, through high school, and only recently began to have any marketable look. But again, the measurements are most important.
In high fashion, the designers are not going to mend there clothes to fit every model for there show. It is much easier to have a “universal” size that models must conform to. That makes it much easier to create ones designers in a specific size knowing that the models are of that universal size. Now the really boyish bodies, or slightly muscular, while fitting into the universal measurements is just the preference of the designer or producer of a particular show or shoot.