A number of people identifying as asexual have never had sex and sometimes are met with this criticism: Maybe they hear, "How can you know you're asexual if you've never had sex?" This is, in fact, the only of the typical asexodoubting questions people get when telling others they're asexual that I have personally received. There are also variations on it skeptical of asexuality in general rather than merely skeptical of that individual's asexuality: "I'm sure you would like sex if you tried it." Or if the person has had sex, "Maybe you just didn't have sex with the right person."
And there are asexual responses. In the first few months I was on AVEN, the impression that I got was that the most common sort of response to this was either that people generally know that they want sex before they have sex or that even if someone has never had sex, they can know that they're not interested. This seems plausible enough. But I also got the impression that some people believed that even without having sex, they could know that they didn't like it. Sometimes analogies to food are given: people claim to know that they won't like this or that food before they try it. This bothered me.
Our imaginations are notoriously unreliable for predicting what something is going to be like, what something is going to feel like, whether we're going to like something or not. Of course, sometimes our imagination is right, but it's wrong often enough that we shouldn't take it to be too strong of a predictor. I don't see any reason why sex should be any different. In fact, there are plenty of people who, when imagining some particular sexual act, find it rather repulsive, but they come to like it after trying it a few times.
But this meant that I believed that people in the asexual community generally can't know if they would like sex or not without having ever had it. I felt that this opinion was subversive with respect to the standard asexual line, so I decided that I should just keep my opinion to myself.
However, a few months ago on Apositive, Ghosts started a thread on this subject (Don't knock it till you try it!), where I found out that my "subversive opinion" was actually one that a number of other people held. To a large extent, this is why I'm writing this series on many of the standard criticisms of asexuality: I have somewhat mixed feelings about the standard asexual responses. I really don't like to be subversive, so I tend to try to keep any opinion that challenges the system to myself except when I particularly dislike the system. In this case, I was afraid of somehow subverting asexual identity. However, I've been coming to recognize that many of these "subversive opinions" are ones that quite a number of other people hold too, but might not always readily express on AVEN.
With respect to the asexodoubting response in this post, my thoughts are similar to those about being a late bloomer or not having met the right person yet: even when it's true that the individual in question might like sex if they tried it (in the right circumstances), telling them that they can't know if they're asexual or not if they haven't had sex simply isn't helpful, especially as a response to someone revealing their asexual identity.
People should never be made to feel that they need to have sex to feel normal. Ever. If they don't want to have sex, they don't have to have sex.
Also, asexuality isn't about not liking sex; it's about not experiencing sexual attraction. People may not be able to know if they like sex or not without trying it, but they can know if they experience sexual attraction or not. And with sex, sexual desire seems to be a big part of what makes it fun for most people. And people know if they experience sexual desire. Also, as noted by Ily on the Apositive thread, some people don't like to be touched. As sex involves quite a lot of touching, such individuals can have a pretty good idea that they wouldn't like sex.