How serious is sexting
that's waaay too general a question: you need to consider and define what you actually mean to ask -
What is the relationship between the two people? Are they married to each other, or to other people? Are they single?
What are the ages of the two people? Is one of them under 18 and therefore is it a criminal offense? It would then need to be reported to the police wouldn't it?
Here's an interesting scenario to consider:
+ Two 16-year-olds can have full-blown consensual physical sex - and that's totally ok and legal;
+ If one of them 'sexts' the other - well that's illegal, and a criminal offense that should be reported to the police.