It's that, and I also feel it is partly creating unfair competition.
If we are to assume that the majority (by whatever margin) of viewers on twitch are male, who do you think most men would want to watch stream their favorite game? A guy, or a girl with her breasts mostly exposed? Men don't have any way to draw in other men aside from their content. Women can have the same quality of content, but add breasts, and they will get more viewers than the men do.