What you see on-line is that people are mentally focused. That's why it's so erotic. Not according to D.H. Lawrence in Lady Chatterley's Lover. D. H. Lawrence would be shocked if he knew that masses of people were engaging in sexual encounters online with their minds. They both are, to some extent.
Humans value both intellectual and physical attraction.
As if that is not reasoning enough, there is the anonymity allowed online. I asked G why he chose the internet instead of real life. No one knows who I am. It is harmless fun. Physical limitations become minimized and almost non-existent.