Autism does not excuse sexual harassment. Pretending it does is a complete ableist trope. As you say, it's a disservice to everybody with disabilities who have learnt, as necessary, to not sexually harass others.
Perhaps some people with different expressions of that "disability" have the capacity to "learn" that they're "wrong". I don't. To suggest that I should have that capacity because others do is... ableist, I guess?
QED
Are you implying that Richard Stallman is so cognitively impaired that he literally cannot comprehend the meaning of the language he uses, or control his behavior? That he has the mind of a child?
If that's the case, he should be assigned a legal guardian, not given a position of leadership and role as spokesman for the FSF.