I think a few things are in operation here.
The first, which the OP mentioned, is the assumption that a woman can't actually hurt a man, because he is strong and she is weak. Thus, ironically, sexism against women backfires onto men.
The second is a conviction that women are naturally non-violent, thus if a woman is resorting to violence, her target must have done something outrageous to deserve it. The OP touched on this with the "morally superior" bit, but I think the "moral superiority" often ascribed to women in this context is specifically gentleness and a preference for resolving things non-violently. (I hope we can all agree that, morally speaking, non-violence is to be preferred over violence unless absolutely necessary.)
In any case, female-on-male abused definitely is not
okay, and I think almost anyone would agree. The issue is less that people think female-on-male abuse is okay, and more that they think female-on-male violence isn't really abuse,
because a) it's not like she can actually hurt him, and b) he probably deserves it anyway.