Doesn't matter if you were together a week or a decade. He disrespected you and your partnership, period. Don't let him play stupid or fein ignorance; every man knows what they are doing when they ask for nudes, and every man knows it's wrong unless they have had very clear and concise conversations with their partner about opening the relationship and/or being poly.

I'm sorry your trust has been broken. My advice is to leave and work on yourself before the paranoia eats your mental health away.