My boyfriend makes dirty jokes about sleeping with a particular girl

Hi everyone! My boyfriend and I are now being 8 years together. I love him and everything about him so much, but recently, he's started making dirty jokes about sleeping with a girl who is a former friend of him. It's really insulting, and with the type of person he is, I really wonder why? When I get annoyed and ask him to stop it, he laughs and says I'm a really jealous girl and it is quite natural for the males instinct to chase multiple mates! I know! it is not funny at all, but he thinks he is teasing me and does not mean it. The problem is because he is doing this so often without considering my emotions, I am thinking maybe time is up for us! I wanted to know if anyone else also gets annoyed about such jokes or it's only me? I am really frustrated and do not understand why a person who declares he loves me should continue this behaviour while I directly said a million times that I do not like these kinds of jokes. And, are there really jokes and not shameful sex fantasies? Thank you.