Why Pornhub is free and sex is not ,because all they want is our attention or something else entirely 

 Marketing companies, Facebook ,Twitter e.t.c ,  all are fighting for our attention because as J.Shridhar  said "Money they can print". Now the question is who are "they" in this story . They are government entities . 

Whichever position you are at either you are some entrepreneur or CEO or common man like me crave drugs or sex or you eat a lot or all of them at the same time ,

 I mean we crave for something or someone. Stomach full & sex then you have no need to look the other way .

I know sometimes it's natural to have sex but only for the purposes of reproduction nothing else.

I am not saying to suppress your urges but when you think about it if reproduction is out of the picture then what else left , nothing , not even pleasure , because at the center of sex is reproduction .