In a like way St. Augustine explains it,
lust occurs out of the soul's inept ability to control the body when temptation exists. This means ultimately that it's not just the body, but the very soul involved in the sin, as both body and soul betray each other.
And this makes it, by the textbook, a mortal sin.
A lot of people don't fully understand what's really going on. When one is not dressed modestly, they ultimately are welcoming the splitting of people's being for the sake of their self gratification.
From God's perspective, this is wicked, and according to His religion it is virtually incontrovertible that nudism and promiscuity are something mankind can handle.
It cannot be rightfully taught or progressed in the world.