Insecure people would download this app, and prematurely 'cancel' all their plans so that they get notifications the moment anyone uses it. This app idea only works with the pretense of privacy until both parties agree, but it is open to manipulation. And once you know that manipulative people are misusing the app, you will police your usage of it, and then we're back to square one, where people are unwilling to communicate.
Yes, same thing happened with mojoupgrade. It's a questionnaire of sex stuff. You and your partner both fill it in. Then you'd see a list of all the stuff you both wanted to try. If either said "no" on anything, the other would be none the wiser. It was often abused (mostly by women) who would answer "yes" to everything so they could see the answers of their partner and give them grief about their wants.