To continue this line of thought a little, let's take a more obvious case. I might say, for example, that I dislike all murderers, every single one of them, and I dislike them solely because they are murderers. I wouldn't call this bigotry, since I have a very good reason for disliking them--namely, that they murder.
On the other hand, your dislike of prudes might qualify as a real bigotry. I am having a hard time coming up with a rational reason to dislike prudes, if by "prude" we mean someone who is excessively modest in regard to anything pertaining to sex. Prudes need not be homophobes. They might be, of course, but they need not be. They might be "sex-phobes"--fear of sex generally. They might go through life without a lot of fun, but such an attitude is hardly immoral.