Men being attracted to multiple women at the same time is considerably normal, as is looking at pictures online of girls. Whether or not you are okay with that depends on you. It becomes an issue if A.) It occurs DURING the relationship, IF you have explicitly stated it is not okay OR B.) He pursues these women. It happened before your relationship; you were not there to set a boundary, but if it still makes you uncomfortable, it's possible he does not share your values.
I'm starting to dive harder into feminism, and I personally no longer want a relationship for mentioned reason. I want an equal partner who respects me as an individual, transcending my appearance. I only want to be in a relationship with a man who values all women in that way, as individuals. You're allowed to find women attractive, there are some pretty gorgeous girls out there, but when you give yourself a quick release (masturbation) to images of women online, not only are you devaluing sex between you in your partner by giving yourself cheap orgasms, you're ignoring the personhood of whoever's behind the silhouette that's arousing you. Men's talk of women they find attractive often reminds me of that of an addict (and I grew up around plenty of those), and I'm a recovering codependent who isn't going to bark up that tree anymore.Also, don't go through someone's phone without permission. Jesus Christ anon.