Education Workforce Council figures showed 13 of 34 staff were barred for such conduct from August 2013 to 2015.
Two more were suspended, while three others were banned for other misconduct involving sexual content.
The National Union of Teachers said it was not a common occurrence in schools.
David Evans, secretary of National Union of Teachers Cymru, said: "We have to be mindful when reviewing these figures that this is a very small number of teachers across the whole profession over a period of a few years. This is by no means a common occurrence in our schools."
He added the union often dealt with malicious and false allegations against teachers, which remained a concern, but said it was right allegations were investigated and any teachers found abusing their positions should face the full consequences.
Donald Findlater, of child protection charity the Lucy Faithfull Foundation, said the changing online environment and social networking in recent years had introduced "massive challenges and risks".
He said some staff got into trouble out of naivety, rather than malice, and schools must ensure adequate training is given so teachers know the boundaries.
A spokeswoman for child abuse charity NSPCC Cymru said: "Schools need to be rigorous in their recruitment and checking processes and provide regular training about appropriate behaviour, making it absolutely clear that crossing the line will never be tolerated."
During the same period, other teachers were disciplined after being convicted of driving, alcohol, drugs, harassment and fraud offences.
Others were punished for assaulting pupils, helping children cheat during tests, failing to achieve required standards, not following health and safety procedures, and failing to investigate bullying.
Nearly 40% of teachers banned from the profession in Wales in the last two years have been struck off for having "inappropriate" relationships or conversations with pupils.