Sex is their only desire not because they are horny all the time, but because they think having sex will solve all their problems and make them a powerful and important person.

Like all good cult/grift sort of things like this, they're both told that sex is the most important thing, and are then taught to behave in a way that makes it virtually impossible to actually get it. It's the carrot dangled in front of them by the group to keep them in line. This promise of "success" that will never manifest as long as they are an incel.