Why do boyfriends follow bikini and butt girls?

Hey ya’ll. my boyfriend and i have been together for 8 years. in the beginning of our relationship he used to like girls bikini and butt pictures and still does but not so much. he also DMed a handful of girls complimenting them. this has been going on from 2015-2019. i had no idea this was happening till a year ago. i was so hurt and disgusted by the whole thing. i’ve asked him why he did it and he said he did it bc he has no idea if we would last. what i don’t understand is why he didn’t break up with me. i asked him and he said he didn’t want to. he’s so different now, compared to the beginning. he’s so lovely, kind and sweet and wants a family with me. i love that he wants to. i just can’t stop thinking about what he has done and it drives me crazy. he also was talking to this girl on facebook- again, he wasn’t sure of us. i want to trust him bc i’m every other aspect of our relationship, he’s honest. from my knowledge. i just can’t let it go. he’s apologized and so far he hasn’t DMed any girl complimenting them. from my knowledge anyway. he could have deleted the DM but I don't know. i’m sad and i’m dying to continue this relationship but I don't know if he’s still doing anything behind my back.
Why do boyfriends follow bikini and butt girls?
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