You're right that a majority of the population doesn't care, I think, because they don't believe it affects them. It's something remote and exotic to them.

However, as soon as it does start to affect them-- say, if someone they know comes out-- if their only exposure to it is through the stereotypical ideas, even if those ideas were presented in the form of fiction, that'll be their mental idea of it that they reach for.

I don't think that any deliberate conspiracy is involved on the part of the general population, so I disagree with the OP's general view of things also-- I think mostly what's going on here is that when people are presented with something that challenges their worldview, they'll often try to deny it, or rationalize it away as something else. This happens with a lot of things, not just multiplicity.

I do believe there have been deliberate attempts to give the stamp of approval to a specific model of multiplicity in the psychiatric community, and to push out all others. Frank Putnam has some studies of non-abused multiples that have never been published, for instance.