First, good netiquette to refrain from sext messaging. So, do not send sexual texts. Given, semi-public.
No Sexting Netiquette
Phones are semi-public. Plus, messages save and share. These things make it a bad decision to send a sexually related text message. High probability someone else will see it. Since, we look for it.
Sexual text messaging is not appropriate because cell phone screens can be read by those around you, among other things. In addition, others steal pics from phones.
Further, many significant others read the messages of their mates. A harmless sext can end a good relationship because of the implications.