Impotence or lessened, uh, performance among cyclists seems to be well documented, though as another post said, it's not universal. There have been about a trillion words written on this topic, and if I were concerned, I'd do research somewhere besides here. Ed Pavelka has written quite a bit about it; you might start by googling his name.
As others have said, lots of us with knee problems have been told to ride to strengthen the muscles, but there are guidelines. One that my doc stressed was to ride in lower gears, not to try to mash in the big ring all the time.