Why is it an either or situation? Obviously telling women to die in a fire or to suck your 'girldick'? is beyond disgusting.
Why is it OK to repeatedly call trans women men or repeatedly harass them online though? (Generally speaking, not about one person in particular)
The former has violent and rapey intent. The latter is just the objective truth, in some contexts it might be impolite but it's a dangerous road to go down to force people to repeat lies.
In the context of discussing the rights and material reality of women and girls it doesn't cut it in most situations to pretend we are talking about women when we mean men. It makes it near impossible to talk about gender and sex coherently, form meaningful analysis and recognise the axis of oppression.
Which of course is why some people want to criminalise calling men men, so we can't have those discussions.