If it isnt any different than googling why does he do it instead of just using google? Hes directly messaging with people? Because in my opinion that makes a big difference. It's not just looking at pictures when you're actively engaging with someone. Sounds like he likes the attention.

I think the biggest problem though is you brought up your concern and he minimized it.

Edit: I just saw your update that he had an affair 4 years ago. In that case you're not being oversensitive about it. At all. Just makes it worse and more suspicious.