If you already have prostate issues and your prostate is not in good health already then drinking too much coffee can irritate the prostate and could worsen the prostate problems.
But if your prostate is in good health then drinking 4 or 5 cups of coffee per day can reduce your risk of developing prostate cancer.
So coffee is not actually bad for your prostate and is actually good for your prostate as long as you don't drink too much coffee and your prostate is in good health condition.
Also if you have BPH or prostatitis then you should cut back on the coffee you drink and the amount of soda or anything else that contains a lot of caffeine.
The best thing you can do for your prostate health and kidney health though is to drink more water than you do coffee, soda etc.