I agree that Joe and Tea Cake's beating symbolizes male dominance over females. The underlying message Hurston means to express is that men constantly feel insecure and thus need to assert their controlling influence over their wives for everyone to see. However, I think we need to remember that Nanny also beats Janie. Perhaps a more inclusive interpretation of the physical act of beating would be protection from society. The theme that it articulates is that in attempting to avoid the harms of society, one merely causes more harm.
This may sound odd at first, but it makes sense once you think about it. Nanny beats Janie because she does not want Janie getting herself into trouble with Johnny Taylor since she does not want her to elope just so she can have an evanescent affair with him. ultimately wants Janie to be happy and secure with a respectable husband. She has good intentions, but is pressing upon Janie values that are not truly hers.
Joe beats Janie because she insults him about his looks and everyone in the store laughs at him. To show that he won't take this insult lying down and to hopefully abate the damage, he slaps her violently. Clearly, he wants to save face in front of society because he does not want others to think that his wife is wearing the pants in their relationship and is controlling him. Casting aspersions on her reputation, he attempts to mitigate the townsfolks' view of him. He acts based upon the values that he thinks society will accept, as to protect himself from the scrutiny of the rest of the citizens.
Tea Cake beats Janie similarly to show that he has control. In fact, the other men of Everglades are envious of how easily Janie takes a beating and tell Tea Cake that they wish their wives were as quiet and subservient as Janie when being beaten. In slapping Janie, Tea Cake conforms to societal values. He is happy that people believe he practically has Janie on a leash, because he does not want anyone else to even think about interfering with their marriage.
In all cases, the characters are succumbing to societal pressures so that they can escape from the damage society has the potential to inflict. However, their overprotective nature arguably contributes to all of their "deaths." (I hesitate from saying literal deaths.) Ironically, because all of them are so concerned with protecting themselves and Janie from society, they lose sight of the point of life. This even holds true with Tea Cake: even though he led a life of adventure, I believe he could have gotten more out of life had he not been so worried about Janie being infaithful.