There seems to be a lot of different explanations given below, but I can't help but think of: It's all of them, more or less. They've pretty much all come up

There seems to be a lot of different explanations given below, but I can't help but think of: It's all of them, more or less. They've pretty much all come up as a consequence to the internet, whether it be better organisation leading more effective communication of how (some) men make unwanted advances, dissemination of pornography or the paradox of choice in dating. You could probably throw on top of that pile greater entertainment choices that are not pornography, making the payoff for sex just too low when you can get instant gratification now.