It does not matter if the two of you think it's cheating or what anyone on this board thinks.

A few people have commented that they've "successfully" done so (kept it hidden from their partner) or whether it "detracts" from their other relationship.

What I think you should ask is: What will that other partner think when they find out?

Sure, you may be able to keep it hidden, but is lies and deception a basis for a healthy relationship? And if your partner does find out, will they be hurt by it? Will they believe it's cheating?

If they would, then you're undertaking an act that has the potential to harm someone you ostensibly care about and who is trusting you not to hurt them. And I'd argue, that if you're willing to risk hurting them for your own gratification, then that's not really caring about them very much.

When your partners find out and are staring at you with the pain of betrayal in their eyes, screaming "How could you do this to me? I thought you loved me?", will telling them "but six out of ten people on bdsmlibrary.com think it's okay" make it all better for them?

If someone believes in fidelity and you've led them to think that's what they have from you, the pain and impact of betraying that is something that will last the rest of their lives and will color every future relationship they have.