I think that all the things you listed are definitely culprits, but I wouldn't be so quick to discount porn.
Anecdotally, I myself and many otherwise healthy, sociable young men I know have pretty serious porn addictions. I really wish there would be more research done on the negative impacts of pornography, especially on young men and teens/pre-teens.
I know typically any anti-porn sentiment is met with "Oh you're just being a puritan", but the reality couldn't be further from the truth.
On social issues, I'm very liberal. Yet so much porn during my developmental years has done so much damage that as an adult I'm just now really understanding the full extent of it, and breaking that addiction is incredibly difficult