I think the idea that futa stuff's a "gateway drug" to becoming bi or gay is absurd.
Anyway, isn't the idea that one can become bi or gay absurd by itself ?
I like what
robozaec wrote, because it depict perfectly the situation.
It's only people who fear sexuality, or deny their own ambiguity, who believe that sexual orientation is a construction, and therefore that it can be influenced in a way or another. Everyone else know who they are, and don't question if something is gay or not, because it have no meaning.
If as a man you have a hard on seeing a beautiful nude man, on photo or in real life, you are gay. Bonus point if you want to be sexually involved with him.
If you have the same hard on seeing a beautiful nude woman, on photo on in real life, you are straight. Bonus point if you want to fuck her for hours.
And if you have the same hard on for both, then you are bisexual. Bonus point if you want them both in the same bed.
It's not more complicated than this. If it happen in real life, or by seeing a photo, then it's relevant. But if it happen through a bunch of pixels featuring a fictional character into a fictional story, then believing that your brain can perceive it as something real tell more about your own stupidity, than about someone else sexual orientation.