What they think of the person you like should not matter. You know why? Because they arent the one dating him, and its none of their business. Sure they might want to look out for ya, and who knows this guy might be a dick (I certainly dont know because you havent given enough information about him or told why they dont like him).... but I doubt this is about him, this is about them.
Girls get very insecure and jealous. All your job to do is to tell them nicely that hey... you like this guy, he treats you well, and if you feel like things wont work youll be responsible enough to break it off. Thats the point where they need to shut up cause after that its not their responsibility to watch over your relationship.

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