Well because on this site, Futa cock =/= gay.
Also, from what I have seen, most gay people are looking for actual MxM content and would not consider something like this gay and would be just as if not more pissed off about the incorrect tag.
Sure you could make an argument that the content in this game isnt 100% straight and I'd agree with you because of where I personally draw the line, but I wouldn't necessarily call it outright gay either because of the above.
I haven't played it but I understand that the MC is pretty much a femboy/twink and only ever gets fucked/used by the Futas/Trans? Well what if the characters were exclusively using strap-ons instead to dominate the mc would you still consider it gay? Most probably wouldn't. The mc would still be a mega bottom but...
Yes, I know it's not exactly a 1:1 comparison but at the same time it's "close enough" in concept that it makes sense.
It's not clear cut and I think the real solution would be to have more expansive tags similar the manga sites.
For instance A "Futadom" tag would be a lot more appropriate just than a femdom tag for this game and a "Futa on Male Only" tag would pretty final as to what to expect.
Anyway the art is good and I am only sad that I probably wont enjoy it because of the futadom theme. I don't typically like futa on male but maybe if the mc looked more girly I could overlook it but oh well.