It is worth noting the "You just haven't done it right yet" is the kind of attitude that one finds contributes to practices like corrective rape, so you can see why people'd have a negative reaction to the suggestion.
But really, the key part people seem to be missing is "Sexual Attraction". Heck, an asexual could enjoy sex on a base "stimulate endorphins" level but they don't experience that Sexual Attraction to anyone. They don't look at a person and go "Yes they certainly could stimulate my Mr Happy!" or "Boy, I sure want their pipe inside my vajayjay!" or whatever it is people think/feel through sexual attraction (I wouldn't exactly know).
Now, some also experience discomfort or disgust at the thought of having sex (in the same way many heterosexuals experience discomfort at the thought of having consensual homosexual sex). Others are merely apathetic to the idea (in the same way that many heterosexuals are just apathetic to the idea of having consensual homosexual sex, they don't have any issue with it they just know it's "not for them"). Some can/do have sex, but it doesn't hold that same primal meaning for them (in the same way a heterosexual could potentially have consensual homosexual sex, but it'd be a dispassionate thing and not hold the same meaning heterosexual sex would to them. Emotionally it'd probably be more akin to an act of masturbation that happened to involve another than 'Sex' with a capital S in that regard).
And of course all other possibilities on the great spectrum of humanity.