I can't agree that defending a loved one or friend is inherently sexist simply because the person in question is a woman. That seems quite backwards to me. Oftentimes, the aggrieved party (regardless of gender) will be unable to defend themselves or appear pathetic if they do so.
Admittedly this was just ribbing rather than full on mean spirited bullying. Rock would have made this single joke about Jada before moving on to his next "victim" so there was little reason for Will to intercede at the time. Furthermore, nearly everyone would have forgotten about the joke within a minute or two of its delivery but now, because Will made such a big incident about it, every one knows about it even if they weren't watching the Oscars. In his attempt to defend his wife, he brought far more attention to it.