Here is the 500px definition:
"Our policy regarding adult content may include any material that is not appropriate for all audiences. While this obviously includes full nudity or sexual activity, it may also include textually explicit sexual content, images containing lewd or provocative poses, strategically covered nudity, see-through or sheer clothing, and close-ups of anatomy that would be inappropriate if shown nude."
Not sure this fits the definition. That's just my opinion, I could be wrong.....