As a man, I think if the artwork is entirely female characters I am more likely to buy the book.
If I were making decisions I would probably weight in favor of underrepresented groups, not sure I could say an exact ratio I would be aiming for. I personally think diversity and quality are related, so I would be more focused on the art being high quality.
My personal take on sexualization is consistency is more important than severity. If the men and women are sexualized about the same, I think that is fine and easier for people to accept. if one is more than the other, people will question it. Past that, I think whatever fits the tone best would be the best to aim for. If you are going for high fantasy, conventionally attractive and unrealistic physiques are probably fine. If you are going for something more realism or gritty then the art probably not.