For me, I think it is because my gf is so uptight and conservative.
Our sex life is nowhere near as exciting as it should be as it just isn't important to her.
She would hit the roof if she knew I'd been sharing pics of her, even though I never show her face.
For me, it is very frustrating and is one way of satisfying my own needs - seeing other men and women commenting on her body. I also kind of feel obliged because so many other men post their women on here and give me pleasure
However like others I do feel guilty so haven't been keeping pics up permanently yet.