As a lot of people have said: while an extreme thing, there isn't anything necessarily wrong with not being attracted to certain demographics, and rejecting them. However, I also wouldn't be shocked if some instances of this are rooted in prejudice. There's no statistic for this kind of thing that I know of, and it's a subjective claim to make, but from what I've seen this is not uncommon.
So for an example on those who reject certain demographics, well, there's plenty. There are people out there who have been traumatised by people with specific features, and it can cause irreparable damage to the psyche. Say for instance, a domestic violence victim gains a phobia/fear/etc of the opposite sex. It may sound a bit weird to some, but it's a lot more common than you'd think. There's entire refuges here in the UK for people with exactly that problem. I wouldn't say this is misogyny or misandry, it's something else. Many people experiencing this issue may never date again, and you know what? Power to em.
I was an unintended victim of trans fetishism by a certain Smash player a couple months back, and it's such a disgusting thing. Never dated the guy, but he would absolutely show his "lust". He was hateful as fuck, and got off on slinging insults at me and seeing me suffer. He was so...warped. I was kind of scared of doing anything about it, or telling anyone, since he had a lot of friends. I did in the end though, and he got thoroughly reprimanded. Him and his mates would defend themselves saying it was just "his thing" but it certainly didn't go over well. Let's just say I was extremely scared of dating people for a while after that. It went away though, and I'm now in a great relationship!
Though as I said initially, it's certainly not uncommon for these kinds of things to be rooted in some kind of prejudice. One look at Twitter or 4Chan will show that. I doubt some people at Incel.co want to actually date a woman, rather they'd have a sex slave or something. Some of the posts there are super disturbing, but "society's rejects" would probably end up that way due to simply not knowing better. On another end of the spectrum, opening Twitter will probably show you a few misandrist tweets getting a few thousand likes, so there's that. Are these ok? Well, it all depends on the context. Is it an offhanded (and very, very distasteful) joke, or are they truly serious about this? If it's the latter, it's very questionable.
But eh, I'm rambling. I'm curious about what people have to say about this.