I think futa is considered it's own sort of genre as far as if it's straight or gay. Plus, they have different kinds of futas too. Usually it's just a penis on a girl, but everything else is female, but I've seen a few doujin where they had testicles too.
Personally, I don't really see it as gay, no mater if it's futa and female, or futa and male. Then again, I'm bi with a huge tendency towards women anyway, so I never really even put any real thought into if it was gay or not for me because a futa is mainly a girl or not.
If you want to get really technical, I've read before that the clit is just an underdeveloped penis anyway, and from what I remember, it's basically the same kind of muscle (not positive on that last part, I might be wrong on the muscle thing). And I've seen documentaries before on girls that have a very small penis where their clit should be, similar to futas, but nowhere near as large as doujin makes their genitals. I've seen videos of this kind of thing, and it for sure wasn't just people that got a surgery to be that way, or anything like that.
So after seeing that, I always liked to see if people change their minds on if futa is gay or not, based on if there are testicles, since a futa's penis is technically a possible abnormality (for a lack of a better way to describe it).