How far is too far?

I've mostly been in monogamous relationships, and I've also been poly. IMO the most important things in any type of relationship are honesty and communication.

That said, I've never thought twice about posting NSFW pics of myself and I've never discussed it with my partners. I would, honestly, have been stunned if, in my monogamous relationships, my SO had found out and objected. By "stunned" I don't mean I would think they were wrong... it's just never occurred to me that they might have a proprietary sense about images of my body.

That stands for publicly posted, non-sexually-graphic images... simple nudes or other provocative photos.

Private messages are another topic. That opens a one-on-one relationship with the recipient that, for me, would require discussion and disclosure with the SO.

In my own relationships, deception has been the most painful and destructive thing I've encountered. It's often really tempting to tell your partner what you know they want to hear, and then do what you want to do under the "what they don't know won't hurt them" theory. Unfortunately, that theory is deeply flawed; what they don't know hurts the relationship, and both people in it.