I really don't get whats so egregious about the dress
The appropriate outfit for people of her status and cultivation is modest.
That dress is meant to show off her body so someone will want to buy her virginity. I like the dress if the person wants to wear it but in this context it's gross. She is being forced to wear a dress that goes against her values on top of being a piece of meat on display.
Imagine if you asked a nun to wear that dress to entice someone to buy her virginity. It's disrespectful and gross.
The context makes the dress icky.