The causes of sexism is another subject. But the first step is to acknowledge sexism is going on in the first place.
This argument could stand if we knew the exact sexual preferences of site users. If tested auditory was e.g. 100% to-male-attracted, what is your guess about “Eric” getting harassed or attacked vs “Julia”?
Also the whole "strong statement requires an extraordinary amount of work" thing is entirely dependant on your outlook. I could just as easily say the opposite statement (that sexisim is minimal) and suggest that it requires extraordinary evidence to prove that. The degree to which you think something is extraordinary depends on your outlook and experience, it is rarely objective.
You could, and it does. But it's it matter of opinion/semantics that this is "the opposite statement".
In context, a specific claim is a specific claim, whichever way it goes. The "opposite" in another sense, is to make no claim, and therefor stick to the default position of allowing for either possibility without knowing which (agnostic position?). There is absolutely nothing wrong with that position, and it requires no additional information, as opposed to "less formal information".
Now, if you claim we need to adopt a specific position, and do so with little information, i.e. without the time/money to do so then you'd have to justify that - i.e why isn't it strongly desired, yet still apparently important, to hold a position on the topic?
To some extent, I would take it with a grain of salt. Anonymous internet accounts can be fabricated and controlled by groups or individuals. This is also something that should be taught, actively.
Do not let apparent social pressure have you concede rationality, truth.
I personally would be very interested to know why "Julia" is being insulted much more than "Eric", but saying "this is because of sexism" is no better than saying that "the sun is bright because it shines".