I think people in relationships punish each other whether they are spanking or not. We say harsh words, we withdraw from the conversation, we with hold sex, we yell, we curse, and we express our displeasure calmly. (This list is of course not complete.) Spanking is only one way of punishing. It happens to be one way that works very well for us. I find the word "punishment" a little distasteful actually. I like to think of it as discipline. We certainly discipline one another in all kinds of ways. In fact we certainly punished each other much, much more when we were strictly vanilla. What we were doing to each other was not always loving.
I found that Jessamy described perfectly how a husband could discipline his wife with out ever spanking her. It seems like a wonderful, warm, loving way of being taken in hand. Some of us go for spanking but that is because we are sexually attracted to either spanking, or spankers perhaps. For me spanking is a wonderful, warm and loving way to be taken in hand.
Unless both partners are perfectly logical, reasonable and even tempered all the time how could you have any relationship at all where there was never any discipline happening? Simply telling your spouse that he was careless with your feelings is a form of discipline. I think that there are healthy ways of disciplining one another and unhealthy ways. What one couple would consider healthy another would consider outrageous. I do not spank my husband, but I certainly let him know when he has been thoughtless or I am unhappy with him. Is this not a form of discipline?