It's no longer hip to be square. Open relationships are apparently all the rage for some couples. But by exploring extracurricular activities with additional partners, the only thing you may be opening up is a can of worms. An open relationship is where a couple decides they want to be together without the restraints of monogamy. Of course, the most open thing about this type of arrangement is the individuals' minds. While it might sound great in theory, it takes a special relationship to endure this type of freedom. Open relationships work for couples who are down with getting down with others. David Barash, co-author of The Myth of Monogamy, tells Men's Fitness, "Having an open relationship can work really well for some people. However, as people, we're also inclined to be sexually jealous of a partner being with someone else, and from a biological standpoint, we're resistant to that partner having another relationship." As with all relationships, even open ones have rules. Here are some of the biggies for couples who prefer to expand their horizons.