For me, it's the logic of "I like women, so two women having sex is great, but I'd also like to see them getting fucked, even though dildos and strap-ons aren't attractive, hmm... ah! I know, I'll give one of the girls a dick so she can fuck the other." And that's basically how futanari was born.
There's nothing wrong with liking it, personally... I don't give a shit if a character is futanari as long as I'm not the one receiving it. I have a Christmas spirit for this (It's better to give than to receive).
And personally, and since this is the second time I've seen the Futanari thread so often, I think it's important to make one thing clear: Futanari and Trap are two different things, and it bothers me when they tag "Futa/Trap" as if they're the same thing, just like with Netorare, Netori, and Netorase.
Futanari is a woman with a dick. A futanari isn't necessarily a tomboy.
Trap is a Femboy. And it's gay, there's nothing wrong with that, but it is gay.