Follow up question:

The Rite of Spring sequence shows the beginnings of earth, evolution, dinosaurs, etc. Was there religious backlash to this segment?

I'm curious, is it ever really presented or interpreted to be a literal orgy? I just watched the video and, beyond a few feminine silhouettes and the topless harpies, I didn't see anything overtly sexual.

There was no religious backlash, as Walt had them specifically in mind when he made parts of the movie.

He [Walt Disney] had decided that the "Ave Maria" sequence should follow "Bald Mountain," partly as an antidote: "We are portraying good and evil." As Walt conceptualized it, the church bells in "Ave Maria" would sweep from the speakers at the back of the theater forward, chasing the demons of "Bald Mountain" offscreen. It would provide, he thought, exactly the sort of slow, somber note needed to end the film—the sort of grace note he lamented he didn't have at the end of Snow White, where he thought he raced too quickly from her sleep to her awakening. He also thought the scene would be commercial. "There's still a lot of Christians in the world, in spite of Russia and some of the others," he had said at a story meeting, "and it would be a hell of an appealing thing from that angle." If it was being shown in a non-Christian country, Walt advised, they would simply snip it off.

Clearly Walt was interested in a Christian appeal, so it wasn't like there was any ignorance on his end in regards to what he was putting on screen.

However, the "Bald Mountain" sequence did get criticized, but for political reasons rather than religious:

"The forces of evil are not shown as the exploiters and war makers," he [Harry Raymond of the Daily Worker] said, "but as a mythical devil on a mountain top against whom human powers are helpless." In short, Walt had abdicated personal and social responsibility for what Raymond[...]called "theology."

And there's also Dorothy Thompson, a nationally syndicated columnist:

[...]Disney and Stokowski seemed to extol the savagery of nature at the expense of man. [...] In Thompson's eyes, Disney's nature was so overwhelming that man had no choice but to subccumb, and like Raymond, she say this as an abdication of responsibility that she obliquely connected to the Nazi terror in Europe. Along with Disney's other recent work, the film was, she concluded, "cruel," "brutal and brutalizing," and a "caricature of the Decline of the West."

Upon release, the film's biggest criticism wasn't the film's irreligious nature, but it's disservice to the classical music. Olin Downes at The New York Times said that "Mr. Disney's noble and highly provocative experiment separated certain lovers of the respective arts rather than united them."

However, audiences seemed to love it, so much so that Walt started working on a sequel almost immediately after it's New York premier. But Fantasia worked almost opposite of what he predicted when he said it would be a "slow money maker, but a big money maker in the long run." It did very well in the first few weeks of it's roadshow run, but for the film's total experience, it required the Fantasound system that was designed for the movie, and it was expensive to install and lug around. RKO wanted the film cut because it was too long, and twenty-five minutes were trimmed off the film's original two-hour-five minute runtime, mainly getting rid of or shortening Deems Taylor's introductions before each musical piece. What killed Fantasia wasn't the criticism, but that the movie was extremely difficult and time consuming to animate as well as the exorbitant costs to make it. All of Disney's films up to that point had cost the studio a lot of money, and after Fantasia the Disney studio was in debt for around $3 million dollars.

Source: Walt Disney: The Triumph of the American Imagination by Neal Gabler

Follow up question: was this scene influenced by what was going on in Europe at the time?