this sounds as plausible as saying that a lot of atheists believe in god. maybe it's a thing? but it's hard to imagine dropping someone in nicea in 380 and suggesting this. words mean things: Christians believe in more than the virgin birth but certainly not less.

why not less? I think many see the "facts" as different. Their approaches to the texts are different, but "Christians" align themselves in someway with a entity called "Jesus". Different sects of Christianity tend to have their own definitions of what they think being a "christian" means and there are commonalities from sect to sect, and even some groups of sects have lots of strong commonalties, but there are still many ways they have incompatible beliefs as well. wikipedia has a reasonable list https://en.wikipedia.org/wiki/List_of_Christian_denomination... and I'm pretty sure some "christians" in one of the sects would go "They aren't christians!" when it comes to other sects in that list.

Liberal Protestantism generally does not consider the virgin birth a critical article of faith. You can believe it if you want but it is not heretical to not believe in it.