IMO Islamically speaking, fiance’ would still be a non-mehram and when for non-mehrams, even the 2nd peek with the sexual intention is haraam, then ‘sexting’ does fall into the category of zina too and is totally wrong!
Socially too, I believe, one should not do this! Alot of times even engagements get broken and usually one of the party gets their ego badly hurt. Imagine anyone could misuse any of the intimate conversations, pictures and videos and exploit the other party involved!!
as for teenagers, I believe parents should keep a check on their children. Spying on them, checking their cell phones is NOT wrong, if it saves their kids from all sorts of evils such as sexting.
I once read a report about a British kid, whose family had to move away because her ex bf exploited her nude pictures all over their highschool as well as on youtube and myspace !!
THIS is a serious concern ! Parents need to be really cautious