Picking up an argument I wouldn't generally make myself, because I think I at least understand it:
Imagine some person, P. P is not very nice and for whatever reason P really hates transgendered people and thinks everything about them is fake and invalid.
P encounters a transgendered person, T, and would like nothing more to call T by the wrong gender to make it clear that P disapproves and to make T unhappy. But P's culture would consider such an action harassment, so P can't get away with that.
So what does P do instead? Maybe P carefully and conspicuously erases T's gender in every interaction specifically calling attention to this de-gendering through its conspicuousness, and ultimately having the same harassing result.
[You could even imagine that T might even prefer the more overt behavior, then at least it would be clear where everyone stood-- and T wouldn't lose support from people who hadn't yet seen enough from P to have inferred the intent.]
I think most reasonable people would agree that someone carefully following the letter of an anti-harassment rule in a contrived way in order to harass someone ... would simply be harassment, and should just be dealt with as such.
I don't think it's that hard to see how some could believe that it is possible to prevent this sort of problem (or at least make giving P a kick in the teeth much easier) by proscribing in advance P's behavior.
I think the problem with that approach is that P's specific behaviors weren't the problem at least in isolation, P's intent was the problem. An effort to prohibit that behavior by blunt rule which is inherently blind to intent will inevitably be full of false positives and be unduly burdensome. Especially so because avoiding a needless invocation of gender has historically been a highly effective way to avoid offending people when you simply don't or can't know what gender to use, and also because the overuse of gender itself offends some people who argue against gender existentialism.
But the idea that there could be harassment that exploits the ability to omit gender seems reasonable to me, even if it doesn't seem reasonable to me to solve that problem by-rule.
Of course, there are much less charitable interpretations of these sorts of disputes. But the mere fact that there are both charitable and uncharitable interpretations possible on all sides is a major reason why this kind of issue can turn into such a mess.