If your wife or husband is cool with it then who cares?
This would be a much different situation if, let's say you where cheating on your partner. But if both of you are okay with this. Then, yes it is okay to have an open relationship (words words words).
It depends on the attitude that you have and if everyone is okay with it. If no one has a problem with it then it should not be a problem. If you're keeping it a secret from your spouse then that is not okay but if you are honest about it then it's less problematic.
First of all why be married? Marriage and the exchanging of vows mean you chose to commit to that one person through sickness and health, through the bad and good times. To have sex and be intimate with that one other person til one or the other dies. By saying you are married and are in an open relationship doesn't make any sense to me. Like why be married? Why not be single? That way you can have sex with whoever you want to. Obviously you have commitment issues. Or maybe you are promiscuous and cant contain yourself to be with one person. Either way at the end of the day I feel that the idea of being in an open relationship while being married defiles the meaning of what marriage and what it stands for.