Feeling insecure...

Just using my partners phone and I noticed on his search history he’s been looking at girls facebooks, these girls are pretty, skinny and basically have their boobs out in their photos and it has made me feel really insecure.. also when we first got together I saw him talking to a friend asking him not to tell me about this girl, I don’t know what about her but I am wondering if he was seeing her when we first got together.. now I’ve just seen her name on the search history, why would he be looking at her Facebook in fact any of these girls facebooks after 5 years of us being together? I don’t want to talk to him about it because I know it’ll start an argument but I feel so down about it. All these girls he’s been searching live in the same city as us so I’m wondering if he’s been with them previously, even though he told me I’m the first person he’s been with. I really don’t know what to think :(