I am just at a lost of words because of how stupid this is...
I mean how could they even think that was a good idea?
It's partly just not thinking, and partly a result of still being fixated on one specific demographic that for whatever reason, reacts really, REALLY strongly to any male homosexual content. Incidentally, it was assumed that no one would call them out on it because their target audience was all heterosexual males that would never even consider Kaidan as a romantic interest and thus would never miss it. It's equal parts of obliviousness, ignorance, dismissal, and erasure.
The reason that Casey (and many others who made this argument) misstepped was the confusion over the word "monogendered" which is how Liara describes her race. The word was often confused with "nongendered" or "genderless", rather than what it actually means - single gendered, and in this case (and in any case where such a species would be successful given what we currently know about reproduction) that gender is female. That gender has to be what we understand as 'female'. Even setting aside her design having all the expected traits of a humanoid female, we know from some animals on our own planet that if a monogendered species would exist, they are going to be girls.
In the end I think it was mostly just a guy with a little residual dudebro in him, who doesn't really understand science all that much putting his foot in his mouth and trampling all over a smarter guy's lore and initial thought process (which was correct). Casey is gone, though, so with any luck this sort of thing will just disappear.