Why Coming Out Doesn't Apply as Well to Autism
The first important difference is that autism is more obvious than gayness. It's relatively common for an autistic person to be identified as such by someone else before they even know they're different. And many autistics can't hide it in some or most situations no matter how hard they try. For example, if you watch Amanda Baggs's videos of herself on YouTube, it's hard not to realize she's disabled. Even my father, who seems pretty NT, was recognized as autistic by an educational psychologist because when he quoted Temple Grandin, he quoted her Texas accent as well as her words.
So it's more common for autistics than gays to be given no choice about whether to come out or not, because autism is more obvious. In fact, it's probably much more common for autistics, especially of the younger generations, to be diagnosed by someone else instead of self-identifying. And then it's on record that they're autistic.
Secondly, if an autistic isn't diagnosed, very often they don't know what autistic is. Most people know the defining characteristic of gayness - being sexually attracted to the same gender instead of the opposite gender. There is no such defining characteristic of autism, or if there is, it's not known. Instead, it's defined by a cluster of traits that are not easy to define or determine. The self-identification of autism basically is 'I feel my mind works similarly to people I know of who are considered autistic' which is certainly not well-defined. And since autism is both less common and harder to define than gayness, most laypeople have a much poorer understanding of how to tell if someone is autistic. So an autistic may know they're different, but have no idea there is a word for how they're different. How do you come out if you don't know what you are?
The normal is less clear too. Most gay people are certainly aware that the norm is to be sexually attracted to the opposite gender. But many autistics think various aspects of their differences are normal. I thought everyone thought the way I did, but for some inexplicable reason were better at handling the world than me. I knew that I was gifted, but thought that just meant having a good memory and thinking fast (and because I didn't remember everything perfectly and sometimes processed more slowly, I doubted I was really gifted).
In addition, gayness primarly affects personal relationships, whereas autism also affects performance in school/work and self-care and things like that. (I know gay people have been fired for being gay, but I'm talking about effects directly as a result of interaction between one's mind and the work environment.) Diagnosed autistics may need to reveal their difference in order to get things like extra time on exams, natural lighting, etc in work or school or else they will have trouble performing. And they may need assistance caring for themselves on a daily basis, such as getting regular meals, although they're less likely to actually get it. In this area, a better parallel are other disabled people.