Masturbating (real people): cheating or normal?

Hi, I’m looking for some​ support and honest thoughts.
I’ve been in a relationship for 5 years. My partner is honestly a very good person — reliable, kind, and we genuinely enjoy spending time together. We have fun together and our relationship has always felt warm and supportive. I never really noticed any serious problems.
But a few months ago we had a conversation about erotic fantasies, and he admitted something that really shook me. He said that about a year ago he masturbated to a photo of a girl on Instagram. She wasn’t a porn actress — just a regular girl. He never met her in real life, but she was somehow connected to someone he used to know a long time ago. He says he basically perceived it as porn content.
Then he also told me that over the 5 years of our relationship he probably had around 50–60 erotic fantasies about women he used to know. Not women he actively talks to now — mostly old acquaintances from university. He’s 25 now and left university when he was 20.
When I heard this, something in me broke. I suddenly felt like I lost my sense of exclusivity in the relationship. Like I’m not the only one in his intimate world.
I started reading a lot of forums and discussions online, and many men say that things like this happen to them in relationships. These weren’t fantasies based on photos — more like random thoughts where the brain just grabs any stimulus.
He has since stopped this behavior, deleted Instagram because he said it triggered impulses, and he fully supports going to couples therapy together.

My question is: 1) Does this ever happen to you? Like, do you sometimes get random erotic fantasies about people you know? Is that normal?

2. Do you see this as just a normal thing the brain does, or would it be a 100% deal-breaker in a relationship?

Please be gentle — I’m feeling quite sensitive about this.