A. Many Americans are overweight or obese. Overweight people are conventionally less attractive and thus less prone to having sex or being in a relationship.
B. Social media permeates all aspects of people's lives. Some people might opt out of having sex or relationships if they didn't want to risk the chance of it showing up on Facebook, Twitter, etc. and getting scolded by Mom and Dad.
C. Poor sexual education in school, usually via fear mongering. There's a bit of satire in the movie Mean Girls (2004) where a sex ed teacher says, "If you have sex, you will get pregnant, and you will die".
I'm not sure if I buy the idea that porn has that much of an impact. Porn in some form has been around for quite a long time.