1. The story is a bit too typical. If I was writing a fictional story about this, this is almost exactly how it would turn out.
2. If you've ever seen someone (male or female) with down syndrome, you know that, unfortunately, they lack a kind of physical attractiveness, without which a healthy person would never consider having sexual relations. In other words - if this guy in the story has any kind of a normal sex drive, it is doubtful that he'd even be able to perform the act due to the fact that his partner would be quite unable to effectively arouse him.
3. If we was, in fact, as freaked out as he says he was when the girl first made her proposal, he would have tried to escape the situation. It makes no sense that he freaks out, and then suddenly chooses to do it?
4. The line "Yeah, I ****ed her" in that specific place in the story seems to be designed purely for shock value. There are many more sensitive, appropriate ways of getting this idea accross. If the person to whom this happened was really writing this story, I doubt they'd attempt to infuse the story with anything resembling entertainment value like this.
Well knows he who uses to consider, that our faith and knowledge thrives by exercise, as well as our limbs and complexion. Truth is compared in Scripture to a streaming fountain; if her waters flow not in a perpetual progression, they sicken into a muddy pool of conformity and tradition. A man may be a heretic in the truth; and if he believe things only because his pastor says so, or the Assembly so determines, without knowing other reason, though his belief be true, yet the very truth he holds becomes his heresy.