If Alicorn had said "I think it's creepy when a guy doesn't know any women", I would not have replied.

Understood, and fair enough. However, it would've been much more helpful if you'd just said that to start with, rather than answering her with a sweeping dismissal.

I personally don't see what she said as that big a deal; I took the "something wrong" part as meaning "you have characteristics women in general will find undesirable" or "you lack characteristics that women in general will require" -- and I see that as a true statement.

If you take this as meaning there is something morally or ethically wrong with you in some absolute sense -- as opposed to merely "wrong" for the result you desire -- then I could see how you might consider it prejudice.

In my case, it's easy to see Alicorn as speaking through the lens of a worldview in which it's simply common sense that of course you would know other people, because otherwise you'd have to be some sort of creep, or at least have a terrible life that's in urgent need of repair. And within the sphere of women's social lives (in general), such an assessment is perfectly logical and sensible.

Your statement seemed to imply that I should "accept as valid" (which I assume means something along the lines of "don't publicly object to")

I mean, "understand why a human being might feel that way and separate your opinion of the behavior from your judgment of the person" - to disagree without being disagreeable, in other words.