Well, you say you "can't count how many times' you've heard people say things about it. How do they know about it in the first place? Not liking sex is fine, but do you go around telling people "Hi, I'm ______ and I'm asexual"?
I don't see why you would feel the need to announce it to anyone but partners and very close friends, if that. What business of most people is it what you like and don't like in private?
You don't need to announce it, it comes up a lot. Being a part of it, you may not realise how much sex - jokes about it, claims of getting it, products selling with it, who's dating who - permeates our culture so completely.
I still enjoy the jokes when there's an actual joke there, and will even make them myself. But many can be summarised as "it's funny because I mentioned sex while making obscene gestures!", like an in-joke - unfunny, but people cite it constantly because it is something they all (theoretically) have in common.
The claims seem base and animalistic, a cruel parody of a society that values you based on how much grunting+ you do.
The ads are condescending and insulting, assuming we're all animals that will buy products simply because of how a person complete unrelated but depicted in the same 10-second bit looks, as though we all think with our (lady-)boners and can be easily manipulated through such.
You don't need to introduce yourself with it. When you're submerged in a culture obsessed with it, people notice you don't interact with it the same way they do.
So many people will value each other based on relationships - if you've never dated a man/woman, and are a woman/man, you must be gay (because sexuality is binary, apparently). That does tend to lead to some of the bullying and oppression that carries.
Then when it is clear you never date anyone, at all, it could never POSSIBLY be by choice. It can only be because you're a loser who's terrible at picking up gals/guys and you pretend you don't want them to feel better about it.
You don't need to announce jack-****. They 'confront' you about it.
EDIT: This post probably sounds really bitter and like I'm attacking you, but don't worry, that's not it. Writing it just brought up some unhappy memories and no post looks good written angry.
EDIT2: ...oh, and God help you if you're male. There is at least some understanding of chastity or 'purity' among women, shying away from sex in favour of an emotional connection. But for a man, sex is supposed to be his end goal. A disinterest in that's almost a social death sentence, and expect a lot of interactions with other men to suffer for it (you have no interest in a large amount of 'male bonding' dialogue/jokes/etc after all). Yep, you're DEFINITELY a loser who's terrible at picking up gals.