help! boyfriend saves girls photos

i heard that he uses them for masturbation so he can imagine him fucking them.. i asked for his friends, guys, opinion they told me its normal for guys to do it, to fantasise about then fucking other women other than his own gf.. i really don't know if this is right or wrong because it has been bothering me for so long, and due to this problem, i always feel fucked up and insecure but whenever i try to talk to him about it he gets mad and stuff and end up being a quarrel, he would say he had enough and stop this bullshit.., but i know that he really loves me and care but this is the only flaw, i don't get it, is it really normal? i'm going crazy and i'm trying to get used to this "normal" (as what i heard ) things guys do, but i can't seem to do it.. What do you think? Is it normal, or is it wrong? and if it's normal, how do i get used to this shit? 😭