Howdy, the purposes of labeling the tentacles as male is more to establish what kinds of males would be into the genre, as I'd wager a higher % of people that are into tentacle stuff are in the broad spectrum of "straight" or heterosexual (yes I'm aware that futa TECHNICALLY falls under hetero as "females with penises" isn't something that would naturally happen and thus isn't something that would trigger the same phisio logical response that seeing a male in that situation would for a sizable chunk of males, though of course I fall outside of that and am actively disgusted by females with male genitalia regardless of term used
Also as far as the strictly male vs strictly female thing is concerned, many people seem to prefer Yuri/yaoi specifically because they do not enjoy seeing other members of there reproductive role in sexual acts.
Edit 2- when I say disgusted I don't mean in a ban it everywhere sense but the same disgust that one has seeing 2 men kiss in public I wish I had links to the studies but regardless of voting patterns/ associates of that nature most people display some level of disgust to that, outside of homosexual males themselves, even if they suppress or don't act on said disgust