Because things don't go as planned.
Sexting is an art which only a few people have mastered after all doing things virtually is not that easy. However, at times the things do not go as planned and quicker than you realize everything turns into a big ball of mess.
Here, I have the confessions of some people who tell in how many shades did their sexting session went wrong.
The word facepalm was invented for serving this very purpose.