They don't understand sex like people without down sydrome do. They get the act because it's animal instinct in all humans, but the actual part that concerns pleasure etc. is something that doesn't come easily. It's just a movie, but sometimes people have sex with those that are not all there mentally because they themselves have issues. It may be because the person with down syndrome is not a threat, sometimes they use people like that for money or government funding, sometimes there might actually be some type of relationship.
I think in that movie, her mother was a drug addict or had a serious alcohol abuse problem? That would explain a lot because she might have wanted to use Sam for a place to live etc., or hide, or money. The sex might have been when she was in an induced state and didn't really care what or who she was doing. Really sad, I know.
There's also another movie like that, with Rob Lowe and Wynona Rider when they are young. You would really like it -it's called Square Dance. Lowe plays a boy with down syndrome who is abused and molested by this lady. He and Winona actually fall in love and she doesn' care that he's got problems etc. It's a beautiful story.
