In other words, the article didn't posit causal relationships between masturbating and testosterone levels and, indirectly, prostate cancer. In fact, a quick Google search would suggest the opposite. From this abstract, it seems that abstinence from sexual activity actually results in raised testosterone levels. I don't think the results are controlled for normal masturbation frequency, which might be important. It might also be useful to track the effects of abstinence over a much longer period than three weeks -- if we assume causation, masturbation would probably only be significant due to its habitual nature. If these things don't matter, however, then the upshot is that masturbation, by reducing testosterone levels, would in theory lower the risk for prostate cancer. As for the study Mike posted, the reason why increased masturbation is correlated with increased prostate cancer risk is probably (again, I'm theorizing here) because genetic factors that code for increased testosterone levels more than offset the testosterone decrease from frequent masturbation. Therefore, if all this back-of-the-envelope theorizing is correct, the highest at risk demographic would be horny guys who don't masturbate.
So, guys, don't be one of those...
Edited to add: Real Men Get Prostate Cancer, a blog on the NY Times written by a guy who is getting treated for prostate cancer. If this were a movie, the tagline would be: "Oh, and my testicles are shrinking."