This is an often-said myth about how things were back then, but it is a myth--at least, for the most part, the way it was described to you. Legally, only a few local laws have ever allowed a man to physically discipline his wife in the United States.
Socially, there is somewhat more truth to what you were told. Men were considered the head of the household and could legally be held responsible for their wives' actions, and as a result, they had some stake in their wives' behavior. Because of this, it was seen as appropriate for a man to be able to keep his wife in line. Using physical force wasn't unheard of. Typically, as long as the beating (I hate to use that word, but I can't think of a better one at the moment) didn't leave any injuries and wasn't done in public it was generally seen as acceptable, mostly because people didn't talk about it. It was considered a family issue unless serious injury was involved.
So there is a bit of truth to what your teacher told you, but it's not entirely right either. It should also be pointed out just because it was more acceptable back then doesn't mean most men did it, just that they could get away with it easier.
The "rule of thumb" myth is pretty accurate as described by others here. It was a myth.
