Are all genital discharges a sign of an std?

No. Men with prostatitis may have a discharge that is not related to a std and in some females can have a "normal" vaginal discharge, but these should always be checked out.
No. No, there are many reasons people can have a genital discharge, especially for women. Not all of those reasons are stds, but it is worrisome for an std and should be checked out...