This one is probably not going to go over well. A lot of people don't like to think about it outside of the goofy context of a Seth Rogen film, and many more people don't like to admit that they do it. They especially don't like to admit that autistic people do it. As I've mentioned before, it's a popular, comforting notion for neurotypicals to categorize autistics as either completely sexually depraved and unable to ever find a sex partner, or completely celibate and disinterested in sex and masturbation. There are people on the spectrum who fit into both categories, though usually less extremely than the stereotypes would have you believe. But in the big middle are plenty of autistic people, myself included, who engage in solo sessions and enjoy them either on their own, or as part of a decent sex life with a partner(s).
The first time I can remember masturbation being discussed and perceived as taboo in a way I was old enough to understand what was being talked about was my Sophomore year of high school. By then, I had already developed a sex drive and masturbated regularly in private. While I considered it a private activity and knew better than to talk about it say, in front of my mother, the topic of "fingering" came up amongst my age mates one day. I didn't know what it was, so I asked for clarification. When it was explained to me, I shrugged, glad for the explanation, and said breezily, "Ah, okay. I do that to myself sometimes." and thought nothing further of it.
But that was only the beginning. One person in the group had a particular penchant for being a loudmouth, and soon, I randomly had concerned members of the school's S.O.U.L (Shout Out Until the Lord, take a guess what kind of group they were) Club coming up to me, and asking in squeaky, concerned voices, "Leah, is it true you masturbate?" with major emphasis on that last word, like they'd just asked if I was the one who shot JFK or peddled heroin. My usual answer, which I mastered perfectly around the 10th time I was asked that week, was usually an exasperated, "Yes, I do. And so do you. Everybody does it. Not just humans. Bonobos, camels, sheep, and bats do it too." This was usually followed with vehement denial on the part of the club member, decrying the activity and me as sinful, and offering to save my soul. I preferred the masturbation to the salvation, so I repeatedly declined. But I was repeatedly mystified at their boldfaced assertion that they did not masturbate, because it was sinful and ugly and evil. Thus my introduction to taboos against sex with someone you love.
Now, as an adult, the topic comes up less in terms of teenage hijinks, and more in relation to my disability advocacy work. I've had everybody, from parents to caretakers to neurotypical friends get uncomfortable at the idea that someone autistic could masturbate. One vivid quote I recall from someone was, "I don't think it's right to show an interest in that. It's just setting him up for disappointment, nobody will ever want him."
I guess it's fair to associate masturbation as a precursor to sex, but that's only exploring one particular facet of masturbation. It can also be employed as a sleep aid, as a stress relief, and it can just be used for the pleasure of itself, with no prospect of it ever advancing to sex with a partner. One example of this is asexual individuals (I've noticed a correlation between being on the spectrum and identifying as asexual, but that's for another day) who masturbate. They usually don't have much, if any, interest in a sexual partner, but many of them still masturbate for any of the reasons stated above. It's a healthy and accessible form of release for them.
And for autistic people who will never get a sexual partner, either because of lack of interest or lack of ability to attract one, masturbation can serve as a reliever of sexual tension. This ties back earlier to what I said about the importance of privacy and alone time for individuals on the spectrum; this allows them a chance to discover, as they reach puberty, that masturbation can be a way of removing stress. And I don't know if all can recall those wonder years, but it's stressful being a teenager and young adult. Doubly so if you are on the spectrum. Why not encourage (Or at least I hope, not discourage) a way for autistic people to learn to alleviate some of that stress, and improve their health? You don't have to buy sex toys for their birthday (For more discussion on that, see Asperger's Syndrome and Sexuality, I believe I recall a chapter was devoted to aids like that in masturbation) but don't violently react to the idea that autistic people need sexual release.
If the topic is handled appropriately, without shame, with the guidance of professionals, with an open mind, and with the opinions of the autistic person always being put first above any notions of purity or propriety, masturbation and self pleasure can be discussed healthfully and safely. It's important to discuss and set boundaries on when and where it is appropriate to do so, but there is no reason why the desire, if present, should be quashed. We have just as much of a right to know and love our bodies as neurotypicals, and in fact, since we are often possess a greater sensory awareness of our bodies and surroundings, masturbation could be a great way of appreciating and managing that awareness.