I've known a few couples in open relationships and with the exception of one couple it has always ended badly. The one couple that is the exception I haven't talked to in over a year and they had just gotten over a rough spot that was caused by being in an open relationship, so I can't even tell you if their still together though I think if they had divorced I would have gotten a call from the wife. Now it is true that some couples can make it work long term, and while these couples are rare they are out there, and from what I've seen and read the only way it seems to work is if you go into the relationship with both partners knowing upfront that it's open.
So why do so many couples who think they want an open relationship fail to make to work? First is jealousy, this is both the most obvious reason and the most common reason. At first both partners will think it's a great idea and be on board, but then after one or both partners start seeing other people one will get jealous of the other and that person may try to hide his or her jealousy because he/she agreed to this.
Second, falling in love with someone else. As time goes on one or both partners in the relationship sometimes start to find themselves having stronger feelings for one of the other partners and start to drift away from the partner that they are in the relationship with. This can lead to people being hurt, relationships being ended, possibly a family being torn apart in the worst case scenario.
As a side note, having one of your partners be a friend that you already have feelings for and they may already have feeling for you is a guaranteed disaster waiting to happen. Once you get intimate those feelings you have towards each other will only intensify and cause trouble for the relationship your already in and committed to.
Third agreeing to an open relationship to make the other person happy. Some times when one partner will ask for an open relationship and say I just need more than you can give me, or something to that effect, the other partner will agree to try and make the person he/she loves happy. This is a recipe for disaster, while the person who asked for the open relationship is out having fun the other partner will resent him/her and bury those feelings because they think this is what they have to do to make the person they love happy. But the feelings won't go away, it will just grow and grow until it's too late and nothing can fix what has been broken. Both partners have to actually want an open relationship, otherwise it could lead to the whole relationship coming to a bitter end.
The biggest mistake you can make is asking for an open relationship if you think your parter is cheating on you and you just don't want to face it. If you think they are cheating on you then you should confront it, not give in to it. Realize that if they really loved you they wouldn't be cheating on you, once you realize that you can move on and find someone who actually loves you. Don't let the amount of time you've been together, or the fact that you're married, or if you have a child together be what keeps you there, understand they don't love you or care about you and move on to someone who will. If you give in to an open relationship just to keep what you have with a cheater, you'll never truly be happy, in the back of your mind you'll always know they cheated and didn't care about you. This is the worst possible reason for having an open relationship, never go into one for this reason.
I'll post some links to sites dealing with open relationships below so you can see the ups and the downs of this form of relationship and be able to make a better decision about how you want to move forward if your thinking about trying an open relationship.
What would I suggest if you decide that an open relationship is the wrong choice for you and your partner? Well first you should sit down together and talk about what you both need, and then do your best to make sure your satisfying your partners needs. Keep the lines of communication open, if your unhappy or feel your giving but not getting anything in return, let your partner know, no one can read minds no matter how much you want them to, you have to speak up! Do you have a child? Or children? The younger they are the most likely they are to get in the way of your relationship, talk to the grandparents or trusted friends and ask if they could take the kid(s) for one night a week so you can have time alone to focus on each other.
Remember make sure what ever your choice that it's the right choice for both of you, if it's only the right choice for one of you then that means it's the wrong choice for one of you as well.