Futanari is essentially a fetish for men, in which they can fantasize themselves in either the giver or the taker. A way to make homosexuality more acceptable in their own mind, because it's a "woman" and not a man.
Not that some women don't fantasize about having a dick, but the psychological rationale is different: it's not about sex, it's about power.
Anyway, it's interesting the term and most stories about it come from Japan, a country where the expression of male homosexuality is frowned upon. It's much like the tentacles: a workaround.
i don't find the claim that it's about homosexuality to be necessarily all that convincing, or at least not that it's the main thing. after all, i could just as well make the sweeping statement that futanari is all about women that experience sexual pleasure and release in the same way as men do, and men find that appealing because it makes them understand the women more easily. futanari are mostly drawn as being particularly womanly in shape which is not something that appeals very much to homosexual men. i'd say femboys are much closer to a "safe" exploration of homo- or bisexuality for men than futanari are.
but of course there is more than one thing going on with futa as well as the recent femboy fad and i'm not sure if something that exists in the intersection of genders and sexes can be easily summed up