This is less of a post, more just a note for me to handily link to/copy-paste from to save me writing it out each time. I've been meaning to write this for some time, hastened by Ily's re-working of the issue.
It adresses the flaws in the argument that, logically, asexuals who masturbate cannot be called asexual, and have to be called autosexual, as follows:
There is 1 operational definition of asexuality- X. someone who does not experience sexual attraction. There are others, such as someone who doesn't engage in sexual activity, but these are generally more flawed definitions, and none of them are embraced by the asexual community.
There are 2 operational definitions of autosexuality- A. someone who engages in sexual-esque activity with themself (ie. masturbation), B. someone who is sexually attracted to themselves.
The problem here is that A and X can both be true, but B and X are contradictory. People are often confused between A and B, meaning that they think A and X are contradictory.
In longer speak: If your definition of autosexual is someone who masturbates, then this doesn't affect sexual orientation, or someone's status as an asexual. They can be both asexual and autosexual. If your definition of autosexual is someone who feels sexual attraction for themselves, then, you're right, autosexuals aren't asexual. However, people who tell you they're asexual and masturbate almost definately (excluding the possibilities of denial*) aren't autosexual by this definition, so therefore they're still asexual.
*and I'm not actually sure whether anyone of this definition of autosexual exists. There probably are some people, though.
Boring but neccesary.
This is a pretty effective logical counter to people who use the word autosexual to invalidate masturbating asexuals. It doesn't, unfortunately, work against people who say "Well, masturbation's just inherently sexual". In that argument, you have two choices:
Try and convince someone that their most regular, fantasy-based, dependable and probably best (from a personal pleasure/achieving orgasm point of view) sex, that's undoubtedly been the most major feature of their sex life since their teens, isn't inherently sexual for everyone.
Point behind them, say "Look over there", then run and hide under a bush until they've gone.