How serious is sexting
Dozy has it right (above) about "brazen conduct". That is the new "catch all" for anything that they didn't have a specific rule about (sexting doesn't specifically qualify as pornia as there is no physical contact).
Sooo much there depends on the elders involved. I heard of a case involving graphic photos of a young couple back and forth (now married) and it was considered something (unfortunately) very common today due to cell phones, etc. and they were privately reproved and went on to be married (in the KHall). I think some BOE might look at it much more "hard line". I think all would look at it much more "hard line" if it involved married persons sexting to others than their mates.
How serious is sexting?
Answer...A JW sister can get pregnant while sexting? :)