Lying about other partners

Hi! This is my first time posting. I’ve been in a poly relationship with my partner who’s married for about a year. I was new to poly and had a hard time adjusting to it. Initially I did for him, but eventually I grew to a “hell yeah” about it.

My partner talked to me about this boy he’s been seeing casually. I’ve been having a pit in my stomach for months that they were dating and he was bot being truthful. One boundary that I asserted in the relationship was to tell if a connection ever escalated.

I ended up finding out that they have been dating for months. When I confronted my partner, he denied it. I guess im just having a hard time reconciling why this happened and Im looking for support. To me it doesn’t make sense because im totally fine with him dating someone, im just so confused why he felt included to hide it.

Edit:
Thank u for the warm and supportive responses. I don’t want this experience to ruin trying poly again in the future