There should be leeway because sexual content is subjective and it gives a few chances to allow users to learn where the line is.
17 is a weird number but having a number is perfectly reasonable to me.
Requiring N distinct reports of a suspension reason would seem to reduce misuses of the reporting system.
The 17-reports threshold might have been found to balance type-1 and type-2 errors, as account removals are costly actions when made in error or as a result of reporting-system misuse.