Just how feminine is it? How odd would it be if a male speaker said it? I have found one example of someone male using, in a piece of fiction: Lest レスト、 the main character of Rune Factory 4 ルーンファクトリー４、 at one point shouts 違ったじゃない regarding something, and his speech patterns aren't that feminine (though not as masculine as they could be) - but I have a feeling that the game's script is nearly identical regardless of gender.
～じゃない is not particularly feminine in Standard/Tokyo Japanese, as long as the ない is relatively short (i.e. sticks to the moraic rhythm) and maintains its low pitch.
I would say that the longer ない is drawn out, and the more rising pitch it is given, the more feminine it sounds.