It is good internet etiquette to refrain from sext messaging, do not send sexual texts because texts are semi-public.
Cell phones are semi-public screens and text messages are saved and shared. These things make it a bad decision to send a sexually related text message. The sexually related text message has a high probability of being seen by someone else.
No Sexting Netiquette
Sexual text messaging is not appropriate because cell phone screens can be read by those around you, among other things. In addition, text messages are easily intercepted.
Further, many significant others read the messages of their mates. A harmless sext can end a good relationship because of the implications.