ED comes from compression of the soft tissue in the perineum. If you've got a big squishy saddle, you're basically guaranteed to compress your soft tissue. If you compress the soft tissue in the perineum 1cm, you cut off all blood flow to your penis. If you do this long enough, eventually you develop scar tissue, and thank goodness for the little blue pill. You want (as was mentioned earlier) all of your weight supported by your sit bones, which will be unpleasant at first, but with the amount mustachians ride, you'll get over it in a week and you'll be fine.
The same thing happens in women, though the effects are less. . .obvious. The solution, fortunately, is really easy. Get a good, firm saddle. Specialized is the only company I know of that does blood flow testing on guy's saddles, and pressure mapping for women's saddles. I've been riding their saddles for most of my cycling career, and couldn't be happier. . They also come in different widths, because people's sit bones come in different widths.