Boyfriend liking other girls photos

I’ve recently went through my boyfriend’s phone and saw that he had liked these sexual/half naked photos of a girl that he used to go to high school with a while back. I immediately felt that it wasn’t okay especially because he liked multiple photos back to back. I eventually talked to him and asked if he was attracted to this person. He tried to explain that he just liked the type of character she portrays in the photos? There was only one pic that I saw that looked like she might be dressed a little out of character, but was still a risqué pic. I automatically didn’t buy his excuse, but I feel like I’m being too controlling.

I basically just want to know what you ladies think about your significant other liking other pics of people especially risqué photos? Do you think it’s okay? How do you deal with it?