My boyfriend always looks at other girls?

I have been in a relationship with my boyfriend for nearly four years now. I know that no one is perfect and that there wiii always be something they do that bothers us but I’m so tired of my boyfriend giving other females attention. He stares at them, he makes constant eye contact with girls he finds attractive even in front of me thinking I don’t notice!!! I’m talking about the same girl multiple times not just always a different girl. We have talked about it a little bit and he denied it, he stopped after that but of course started doing this again. For example, there is a girl at the gym he finds attractive so I catch him staring at her a lot and I catch her flexing herself for him. Sometimes he comes home and TALKS to me about her? Like she was doing this and that etc... this has happened a couple of times until I told him that I really don’t care.

I DO NOT understand WHY he is doing this. I find this extremely unattractive and in my eyes he is becoming less attractive while other men are becoming more attractive to me to be honest!! I know I have decide what to do but I’m curious what other thinks.. is this a common problem? Thoughts?
My boyfriend always looks at other girls?
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