If you are only sexualizing one type of character, then you are showing bias towards viewers. When this becomes the norm for most art in a society, it says that type of character (and the people it represents) is primarily there for sexual gratification. And when you do that to women, you create a culture where women are objectified as sex objects and little else is given value. It's inherently reducing a major population into nothing more than the desires of the dominant. I.e. oppression, and specifically in this case: the patriarchy.

It’s only bad when all or nearly all characters are sexualized.

I wouldn't even go that far. If you look into porn or porn games you'll see nearly every character sexualized in different ways depending on the intended audience. Romance novels are an example as well. And when the point of the media is titillation and sexual gratification; that's both good and expected.