As a women, I know first-hand that rape culture does exist. However, this interaction is not an example of how it starts! I have two boys, a bit closer in age than the authors. There have been times when the youngest has bitten and been in trouble, there have also been times I have seen my oldest, grab a toy and shove his brother to the point my youngest bites. To which we say "you should never bite but honestly if you pick on someone you have to expect that they'll fight back eventually". 

Maybe Liam was getting beaten up, maybe it was his only defense.  We usually ask our oldest about what happened (greater vocabulary) and then our youngest. And then sort out that biting is wrong but discuss what the oldest could do in the situation (sometimes walk away). Again, both are boys, but the oldest has more responsibility to diffuse the situation. Perhaps the husband asked because he felt that the older child had control of the situation, not because she's a girl.