The reason why people with autism/aspergers have difficulty getting into relationships is simply because of the nature of the disorder itself. It's hard to talk to people, it's hard to read people, being social feels like a foreign language to many of us. It's like trying to play a game without even knowing the rules, you're going to make a lot of mistakes trying to figure out what the hell you're supposed to do. And how are you able to attract a girl, if you can't even talk and form a connection with her?
Not to mention that a lot of people with autism have difficulty regulating their emotional state so they're more likely to suffer from anxiety, depression, and even suicidal thoughts. All of these things make it harder to form a connection with a girl.
The high unemployment rates and higher likelihood of living with your parents for those with autism well into adulthood doesn't help either; many don't even drive and they're in their twenties. Poor executive functioning skills makes it harder for many people with autism to live independently and function in society as an adult on their own. And girls past a certain age usually don't like guys who are unable to take care of themselves on their own, are unemployed and still live with mom and dad past the age of 25.
Also a lot of people with HFA (high-functioning autism) neglect their appearance in general. They don't shower, they don't shave, they wear baggy clothes, all of the stuff that any normal human should do if they want to go out.
If a woman is interested in a guy she will admit she is interested in him. Dating/relationships are black and white and there is either instant attraction/love at first sight or there is no attraction/not interested. Where there is no attraction/no interest, move onto next! There is no subtleness it is obvious interest or there is no interest. I am an Aspie and I see the world in black and white I know that my theory is true because most non-Aspies I know have felt instant connection, love at first sight. I have never felt instant connection and never experienced love at first sight because I am an Aspie!