Let's start by defining CNC, Consensual Non Consent, as any practice or play where all parties consent to the act in question but may appear not to do so. Now some people look at this and automatically assume rape fantasy and while that does fall under the CNC umbrella there are many other things that do as well. Examples include somno (sexual activity while asleep i.e. waking your partner up with sex) and "free use' subs.
CNC is a prominent aspect in a number of different dynamics including "vanilla" relationships. It can be something that adds a bit of spice and shakes things up, an occasional departure from the norm or a constant on-going understanding between partners. However it develops, with whatever frequency it occurs, the most important aspect is communication.
In order to be classified as CNC rather than NC, both parties must have the opportunity to consent. Without that communication... well, things can go... badly.
Example 1: Derek and Shante have been dating for awhile and Shante has expressed to him that she likes the idea of being woken up with sex. Shante spends the night one night and Derek wakes her up with sex the next morning. Shante was asleep when the sex was initiated and did not consent to it, but both parties enjoyed the sex and nobody felt uncomfortable or upset. This is CNC.
Example 2: Derek meets Shante at a party. Both of them have been drinking and they go back to his place. Shante passes out before anything happens. Derek decides to go to sleep as well, but the next morning decides to wake Shante up with sex. Shante's memory is fuzzy from the alcohol and she doesn't know where she's at or who she's with. She is not comfortable with the situation, but is afraid of saying no or trying to stop him. This is NOT CNC.
It all comes down to communication. Let's look at another example.
Example 1: Morgan and Alexandria have an established Dom/sub relationship complete with safewords. Alexandria has expressed on numerous occasions that she wants to be "forced". One night, Morgan stages an "attack" in an attempt to fulfill Alexandria's fantasy. Throughout the experience, Alexandria says "no" and begs her to stop. She does not use her safeword. When the experience is over, Alexandria lovingly thanks her Dom and they talk about the things each liked most and least. This is CNC.
Example 2: Same scenario, but in this example, Alexandria uses her safeword, and Morgan doesn't stop. When the experience is over, Alexandria is upset about what happened. This is NOT CNC. This is rape.
It should also be mentioned that CNC fantasies, in all their various forms, are quite common for guys, girls and non-binary swirls. Some of these fantasies are the result of trauma. There are a number of survivors that find exploration of a CNC kink helps them process traumatic events. Sometimes they like the ability to create a "controlled" experience similar to one that was outside their control when it happened. Sometimes they just find it to be a sexy or fun idea.
It doesn't matter what one's reason for engaging in CNC, the bottom line is simple - if you communicate with your partner(s), CNC can be a fun and enjoyable experience for all parties. But without that communication, it's just rape.