It's something that's definitely increased in the last 10-15 years with the rise of social media, texting/sexting, and cell phones. It's really easy for someone to get in touch with other people, look up ex's, and communicate quite secretly. I mean a person can literally go in the bathroom for 30 seconds and take a nude picture and text it to another person. It doesn't take much nowadays.
So although it's not a first date topic probably, if you're in a fairly long-term relationship with someone you at some point want to discuss what you both consider the boundaries to be for cheating. You don't want to not discuss it and find out later the person you are dating didn't consider sexting to be cheating but you do. Generally you'd think what qualifies as cheating is pretty morally straightforward, but it seems like it can be a bit subjective.