My bf feels like I cheated on him

My boyfriend and I have been together for about 3 years now and live together too. We have a happy, loving relationship and do a lot for one another. We both see our relationship lasting long-term. We're 24.

Anyway, to make a long-ish story short, a couple months ago my boyfriend went through my phone (I know this is super controversial, but we've both always known each other's phone passwords and allowed for that as we both run anxious in relationships). He was extremely mad for a couple of days after and initially I had no clue why - I knew I hadn't cheated or done anything unfaithful. Turns out he came across some pictures I had stored in a hidden picture-type app that I haven't used in years. Those pictures were naked photos of my most recent ex, whom I've had no contact with since we broke up about 4 years ago. I had **no** idea those pictures were there, and had forgotten about the app completely. Not once in my current relationship did I look at those pictures, let alone do anything beyond that. I very adamantly believe in deleting explicit pictures as soon as a break up occurs, so I felt embarrassed and grossed out that they were there.

Anyway, to my boyfriend, that means I cheated on him. Now, let me be clear - I've never flirted with, talked to romantically, or touched a single man while in this relationship or any relationship ever. I have never cheated in any relationship and I am vehemently opposed to doing so. My boyfriend has called me a cheater multiple times and told me the friends/family he has told think I am a cheater as well. This is obviously super upsetting to me. Every time I tell him I did not cheat on him, he tells me I did and that saying I didn't is invalidating his feelings. I think he absolutely believes that I was looking back on those pictures and wishing for my ex, which isn't actually true at all.

I do not think it's fair in any way that I get labeled a cheater for the rest of my life when I did not cheat. Still having those pictures was wrong, absolutely - however I **never** looked at them and was shocked to find out they were there.

We're in a couples therapy, btw. This hasn't come up yet though.

I'm looking for advice on how to approach this with my boyfriend. I don't want to invalidate his feelings (this literally did hurt him deeply, and I care about that), but him calling me a cheater repeatedly isn't okay with me. How could I approach this in a validating way that also communicates my own feelings?

P.S. I know common advice in here is often just immediately to break up, but I don't want to or intend to do so, so I'd really appreciate advice other than that. Thanks :)