Hmm, thinking about this some more. I think cheating probably *is* too strong of a word, since this was someone you were already intimate with by mutual, open agreement. Instead it would be more fair to say that what happened was rules/boundary/agreement breaking. If it had been with someone your partner hadn't known about or had expressly forbidden sexual contact with, then it would have been cheating, whether you were mono or poly, and whether it had been deliberately hidden or not. Just some philosophic/linguistic musing on my part...
Anyways, the thing about triads, to my mind, is that they always have the possibility of becoming vees. Like, let's say you two hook up with a chick named Sarah. Sarah loves you both and you both love Sarah, and all is well in triad land. But then two years down the road, Storm and Sarah fall out of love, while Rain and Sarah stay love. Do Rain and Sarah have to break up because a triad was ok but a vee was not? What if they're too in love to countenance breaking up? To my mind, it's dangerous to start a triad when a vee is emotionally unacceptable, because life is just too unpredictable.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.