No this is not how most people see it if you mean your crush being stolen. That is more friendzone territory then NTR. There is only a handful who do. It only becomes NTR if you are in a relationship. Also NTR can happen without SA or blackmail easily, it basically is cheating while they steal the partner. This can happen through many ways.
(edit) Basically if this was the case it means if you have a crush IRL, but she starts dating some1 else and she friendzones you that already is NTR by that logic. Which means NTR wud be fairly common.