People should definitely be able to have multiple relationships. If the partners know about the other relationships and are ok with them or has their own relationships, then no harm is done. Monogamy isn't for everyone and it doesn't make sense to try to dictate other people's lifestyle choices for them when they aren't hurting anyone.
I had feelings for two girls at the same time and went nuts trying to decide who should I choose until I recently became polyamorous because I don't think there is a reason not to have both. Polyamory was a very unorthodox method so I researched and I realized that nonmonogamous relationships is completely natural and are the future of new relationships. There are many benefits that multiple relationships have including honesty, non-existense of cheating, and more partners means more love. Polyamory is not even immoral since many Christian saints were nonmonogamous and the Bible says nothing against threesomes. If you get the chance to have more than one partner, sexual or otherwise, take it. Studies have shown that a three-parent household benefits kids much more than two parents both money wise and more people to babysit. Just remember that cheating is only when its behind someone's back or without their consent so be honest and open without everyone in the relationship. Thats real commitment, not marriage.