I agree. In real life, people like Brittany would not have such friends. People don't like/enjoy hanging out with people who are phenomenally stupid. Most people I know would avoid conversations/contact with such people. But TV is what it is and most casts have that one incredibly dumb friend that the rest of the cast still hangs out with despite having no respect for their intelligence. Artie having a sexual relationship with her is also a little creepy, because, like you said, it's almost as if he's banging Becky (the girl with Down Syndrome), which would completely incite the general public to disgust and anger.

Emma's revelation was certainly surprising and highly improbable, but a little more within the realm of possibility. I agree that it did seem as if she had broken barriers with Carl and that physical intimacy shouldn't be such a bizarre terror for her.