Can someone please explain to me how guys don’t see following bikini girls is a big deal?

good afternoon,
my boyfriend follows a lot of bikini girls on instagram and it bothers me so much. it’s been going on. for years and i always tried to not let it bother me. recently i’ve let it get to me a lot worse then usual. i’ve asked him and he said he would stop if i change he will then a few mins later he said he won't stop. i don’t understand why guy follow bikini girls for i dont understand. I've told him he should find someone who doesn’t care not who he follows he said ok. so i truly want to know why do guys follow bikini girls for i don’t understand what is the purpose? like for a future husband i just don’t want to care. i know it’s not a big deal but for some reason it’s a big deal to me. but i don’t want to care anymore. any please have any advice on how not to care. i do not want to care if my boyfriend, fiance or husbands follows any bikini girls. for the future i dont want to care.
Can someone please explain to me how guys don’t see following bikini girls is a big deal?
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