Oh, good, this old argument. What the hell, I have a few minutes to kill:
Futanari, roughly translated from Japanese (where the word comes from, in case that's a surprise), means futa --> "two" and nari, "figure".
This is reference to a dual nature or sexuality of a person.
Outdated terms for this could be considered hermaphrodite or intersex, but the general thrust of it is describing a person with male and female genitals.
So, in less professional sounding words: One genital: cock. Second genital: pussy.
Both sexes. Not one. Two. 2. Twoooooooooo.
Over the last however many years the term has shamelessly appropriated and repurposed to describe what is essentially a newhalf, trans, shemale, trap, dickgirl etc, all lumped into one. (For the record, these are not all the same thing.)
See, in this day and age everybody is angry at everything, and a lot of those above words can trigger people. 'Futa' or Futanari is just foreign and fancy enough sounding that it can slide in there and be referenced (not everybody digs it regardless, as anybody could see eyeballing even the board here), so it's been lazily slathered all over the damn place due to its ease of use. It's lazy, but more importantly, "safe" to write all sorts of things under that umbrella of a word. And so it goes.
Which muddies the waters and makes it difficult to even track down ones preferred type of "futa".
For example, hunting for images for a traditional or true futa often requires the term "full-package futanari" to get both bits, and even this bit is confusing, because a proper futanari can have either external balls or them being visually absent. Where are they then, and how does the futa produce sperm?
Meh. Some artists and minds go with them being internal, others handwave it away as sexy hentai magic, whatever works. Sometimes the cock is always on, sometimes it grows like a super powered clit ala Bible Black or Angel Blade. At the end of the day it's really all about the creators imagination and design. Which of course, means they (the creators) can point excitedly and go "AH HA! Yes, my design is to have them with only a cock and balls below! Suck it!"
Yes, it absolutely can be a choice. I'd argue it's also absolutely wrong, but it's essentially a subject of pure fantasy, after all. Personally I find true or full package exceedingly hot and yields a wonderful variety of positions, situations, and options to explore, but people are indeed built different.
In short. A lot of folks are using the term incorrectly. A lot of folks don't care in the first place. And language and culture "evolve" steadily, so it can get pretty confusing when several generations latch onto words and use them casually with entirely different meanings.
And even if you give people all the information, it doesn't mean it's going to be read or accepted in the first place. I mean, this is a pretty disgusting wall of text.
What, reading on my pirate porn game forum?! We'll have none of that nonsense here!
And now you know. (And blowknowing is half the futanari battle.)