Honestly, it's not hard to understand at all. Just picture a scenario in real life: You're dating this girl you really like, and she tells you she'd fuck a trans person (or anyone she finds interesting for that matter) if given the chance (while still dating you). I don't know you personally, but it's safe to say the vast majority of men would have issue with this. The though counts, even if the action is not actually done.
I think the issue here is much more about "sharing" and "NTR" than "futa". A lot of creators tend to forget a lot of people get invested in these characters (some waaaaay more than they should), and they don't wanna see the version of the characters they have in their head to be destroyed.
If you're hell bent on adding this content, I'd advise not involving any of the main girls, and doing it with a minor side character. Even so, there will be backlash. Involve any of the main girls, and there will be a shitshow, optional content or not. If you are fine dealing with that, its your call
Right...
What I don't understand then is what's hard to understand in "It'll only happen if you toggle this option on, otherwise it won't even be mentioned."
Think of it as "parallel universes". In one universe the option to have sex with the futanari character is activated and in the other it isn't.
You live in the universe where this option is turned off, why would you care what happens in the other universe?
Also, if like you said, people get invested in these characters way more than they should, than that's their problem.
I shouldn't change what I have in mind for my game just to please someone that invested way too much of their affection into fictional characters.
Besides, when you try to please someone, you end up pleasing nobody.
I'm already doing a lot by making everything completely optional.
There are some lines I won't cross though, mostly because they're stuff I don't like myself, like NTR, any kind of dom stuff or y'know, the very fringe stuff.
Meanwhile, I'll keep going with my original vision of the game and, if people don't like it, I'm sure they'll find other games they can invest their affection into.