We should absolutely be teaching teenagers that they can say no for whatever reason, that is a complete given.
We have a long, long, long way to go to achieving this end position for all teenagers in all situations, and young people are still in the thick of confusing encounters that they don't know how to handle well. They are the collateral damage in our society's present inability to hammer home this message.
Asexuality is not a diversion to this, unless you wish you persist in an authoritarian direction of denying the expressly owned identity of others. It is legitimate for an individual who has enduring experience of lack of sexual attraction to others to declare they are asexual, in the same way it is legitimate for an individual to declare they are a lesbian etc. It does not detract from anything or anyone else.
When guys ask me out and I say I'm a lesbian (I am) are you saying I should not be allowed to say that, and in fact I should just say no? I mean I obviously can completely just say no, but saying I am a gay woman adds context to my refusal and it also raises awareness that lesbians do actually exist, which seems to be forgotten a lot of the time, and is a huge problem for gay women. We are sidelined, forgotten, we are often read as heterosexual. I am entitled to proudly assert my identity in these situations; an asexual person is likewise entitled to proudly assert their identity.
What it does do is create a broader understanding that the identity and the experience is legitimate, which will support a societal understanding of the various different experiences and identifies we all have, and reduce the likelihood of an automated reading of a situation as a likely heterosexual woman who is likely interested in sex. Which in feminism previously used to be considered problematic, but in this aping the patriarchy world of FWR Mumsnet, is apparently not any more.