text only Cyber sex with underage girlfriend illegal?

boyfriend and girlfriend have been dating for 5 months. He is 19 and she is 15 1/2. they both exchanged sexually explicit messages via cellphone/facebook. Both of them are sexually inactive and have made it clear to eachother and their parents that they would not engage in any physical sexual activity until the girl is 18. There is evidence of them discussing and agreeing on the matter in many of their chats. Non of their messages contained photos or videos of any kind. Text only. And he was not coercing her into meeting up for any sexual contact. Just sexual fantasies between them.