Author here. First, I didn't claim to by a "raging introvert". But I was more introverted when I was younger and I believe I'd still be like that if I had already found a partner. Also I'm probably using the terms extrovert and introvert in a very sloppy manner to just describe the amount of socializing. I might have better chosen other terms.

Second, I didn't claim that people are only loyal because of lack of alternatives. I apologize if that came out offensive. But I still believe that people are more likely to cheat on their spouse a) the more alternatives they have and b) the more they enjoy the process of dating and flirting.