Listen up you retards because I'm about to enlighten all of your weak minds
Futanaris are straight, just like Pegging is straight.
Why?
Let's begin to look at what a Futanari is:
Futas are essentially biological women (100% female) who for one reason or another have cocks (often times perfect pristine cocks or huge inhumane ones)
Futas came up as a way for Doujin artist to pair up female characters and have them be responsive during sex (a strap on may please your partner but I doesn't do anything for the one pegging)
In other words Futas are a fantasy about women being able to penetrate while also feeling it.

Now why are they straight? Well implying we are talking about FUTA on male they would be straight because sexuality is very clear cut in these aspects, when a man has sex with a woman they are having Heterosexual sex. There's no room for interpretation here, whatever you believe or think a man should do during sex is irrelevant. If you have sex with a woman you are engaging in straight sex, and if you actively chase after more straight sex you are indeed a straight man.
And since we stablished before: Futas NEED by definition to be biological women.
Also you can't try to use real life "equivalents" to futas in order to prove they are gay. Because such a thing doesn't exist. Futas have exaggerated dicks, often inhumanly huge or extremely smooth and perfect, also trannies in real life are MEN so they can't be futas by definition.
So yes, Futas are straight because the fantasy relies on the involvement of a real woman, something straight.
And if you want to argue that the OBJECTIVE definition of straight sex is wrong and that sexuality is more about behavior then I'll assume you are a tranny since that's quite literally the arguments they employ to justify their mental illness.