Same as a chair my arse.
Well, that was true of Lucian Freud. He would spend the same amount of time and effort painting picture of a horse's hindquarters, or a cushion thrown on the ground, or a patch of leaves where his dog had been buried, as a human being. He says somewhere that he's always treated the head as 'just another limb' in his full-length portraits. I imagine he felt similarly about sex organs.
And he paints almost all of his full-length nudes lying down in similar poses, so it's not that he singled his daughter out for this one.
(He drew his mother the day after she'd died, too.)
And given that he was a charismatic monster, and had at least 14 children two from his first marriage, and 12 by various mistresses, many of whom only found out about one another later on several of his daughters (many famous in their own right -- Bella Freud, Emma Freud, Susie Boyt) who've talked about it in interviews mention competing for his attention, so I can imagine that being part of the attraction.
Personally I find Balthus's pubescent girls far creepier.