If you're in a relationship, should you stay friends or stay in contact with someone that is always more than just your friend whenever you are single?

My boyfriend has quite a few friends that are women. He tells me the only time they are ever more than just friends is when he's single but when he has a girlfriend they become only just friends again. Our relationship took a few breaks, during all these breaks he's pusued these women and they carried on being more than just friends during the length of these breaks. Everytime weve got back together to work on our relationship he stays in contact and maintains his "friendship" with them. If i express my concern, he tells me im being insecure and theyre just friends. The only time theyre more than that is when he's single.
If you're in a relationship, should you stay friends or stay in contact with someone that is always more than just your friend whenever you are single?
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