Not just the midriff -- that was a poor example. I can think of a more scantily clad male character like Kuja for example as someone already pointed out. Pretty much every main character, male or female in this series is exploited sexually. They know they have to appeal to straight fangirls and gay fanboys as well. While I agree with your argument about Lulu, I always saw her large breasts as more of a symbol of her maternal role in the group rather than it just being about ta ta's.