I had asked bluebird for this link (Thanks for bumping it, bluebird), and find it intersting that most of the bicycling discussion revolves around incontinence, but when I searched on Google (KW="prostatectomy bicycling") it brought up several references to an association between bicycling and ED. Here's
one of them, with the related paragraph from it as follows:
"Bicycle riding is widely recognized to be both beneficial for the environment and an excellent cardiovascular exercise. However, population studies have shown that riding a bicycle for more than 3 hours a week is an independent risk factor for ED and generally occurs in a younger and healthier population. Single-vessel arterial revascularization surgery may provide an opportunity for natural, spontaneous erections in men who experience ED as a result of bicycle riding. Munarriz and colleagues of Boston University performed a retrospective analysis of the records of 79 men with ED associated with bicycle riding who received penile revascularization surgery from 1998 to 2002 with a single anastomosis between an inferior epigastric artery and a dorsal artery. They demonstrated significant improvement in erectile function and intercourse satisfaction and a corresponding improvement in overall well-being. This therapy should be considered in a carefully selected population with isolated arteriogenic ED.
From another article:
"Bicycling. Studies have indicated that frequent bicycling may pose a risk for erectile dysfunction by reducing blood flow to the penis. The greatest risk is in cyclers who sit upright while cycling. In addition, a 2004 report in the Journal of Urology found that long distance cyclers may reduce their risk by riding a road bike instead of a mountain bike and by choosing saddles without a cutout."
Seems from that articles that there is an issue with blood supply, even without prostatectomy, and that there are other good reasons to avoid it in this situation. (I'm not an avid biker, but it does come in handy sometimes, and it looks like the wisdom now is to avoid it, even if the incontinence is not an issue.)