Oh my god, no. It's a thing made up by prudes, talk-show hosts, and insecure, jealous idiots. It's a ridiculous myth. There is nothing wrong with having best friends outside of marriage, there isn't even anything wrong with having crushes outside of marriage (not that you said that). You should ask him what he means by that, and don't feel like you have to break off with your friends over it. If he's feeling jealous or insecure, then you both need to talk about and work out where you both stand, and what you both want. But forget the soap opera/bad Oprah-episode/jealous insecure teenager advice that you're getting from the people above. It's ridiculous. Go ahead, and be a smart person who talks to their partner, has a life outside of marriage, and knows what she does and does not want.