Wow, that really is a stupid argument. If there's no clear top, it's obviously "female with male".

Yeah, that was in fact the issue; their tagging system required mixed-gender pairings to explicitly specify which gender is topping, with absolutely no provision for "neither", so they just kept going in circles trying to figure out which one was the top.

Did they think nobody would ever draw any of the various sex acts that don't have a clear top/bottom dichotomy, or did they assume that all of those sex acts are necessarily gay?

#or worse. did they assume a secret third even dumber thing. (via @wingedcatgirl)

I'm afraid it's the secret, third, even dumber thing: for acts which lack a bright line between penetrator and penetrated, they had precedent-setting rulings. I don't rightly remember most of them after so many years, but it was stuff like, in a mixed-gender handjob, the one getting their penis jerked is the top, unless they're tied up at the time. Fascinating stuff. For whatever reason, mixed-gender frottage turned out to be their system's Kryptonite.

I am going to assume their penises were both similarly sized because otherwise the most logical option, taking those precedents into account, is that whoever has the larger penis is the designated frotting top.

That argument was raised, yes.