* Insane portion sizes when I eat out.
* If there is a smaller size it feels not worth it (it's not that much smaller, but the cost is still dis-proportionately high, adding a psychological barrier to ordering less)
* I hate leftovers, but I was also given that 'starving children in Africa' speech as a kid... I hate leaving food behind.
* I have a 'desk job' and thus don't get enough exercise.
* American culture is predominantly a car culture, our cities, suburbs, and rural areas are built to that standard.
* I wasn't, really, at all trained in making HEALTHY food as a kid.
* Making healthy food takes some time, or at least a good bit of planning.
* Making food for one means I can't keep the best healthy ingredients around. (Fruits, vegetables, to a degree bread) Even the smaller store sizes are at high risk of going bad before I get them, and they have a cost high enough to make me lust for prepared foods.
* Most of the prepared foods are made to sell on taste, and thus have a lot of what isn't 'healthy' for us.
* We've also, apparently, been on a war against fats, when their common substitute sugar is actually a more insidious enemy.
To C: Probably people know more about sex than they used to, not less. Anyway sexual eduction leads to the first sex you have being later in life.
Porn certainly is more accessible and widespread than it has been.
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.