It’s one of the main causes of fights and breakups. Remember, they’re with you. If they wanted to be with someone else, they wouldn’t waste their time. They’re going to have friends of the opposite sex [or same sex if that’s how you roll]. Telling them they can’t be friends with someone they knew long before you came around just because you get a little jealous, is unfair and controlling. You have friends like that, too, remember. Let them know your concerns, and leave it at that.