Hmmm... I know exactly how you feel. The guy I am newly dating wanted to buy a painting last night of a sexy naked woman, and I frowned upon it because of my own insecurities. He didn't buy it because I was down on the idea. He asked if I felt threatened by it and I basically admitted that yes, I did.

So I get where you're coming from.

Thing is, the two of you are trying to establish trust right now, BUT at the same time, you're not exclusive. So because you're not exclusive, he does have the right to like women's sexy naked pictures, to flirt and even go out with other women. I don't know how you work on building trust when you're not a couple and are not in a committed relationship yet. At this point, he has the freedom to do as he pleases. Maybe you two should try being a couple and work on building trust with a commitment? I don't know. :/ Sorry if I am no help.