This strikes me as a bad idea. By my measurement, during erection, the penis contains 6 ounces or more of blood more than it does when flaccid. This being an appreciable fraction of a pint, the amount drawn during blood donation, it is reasonable to suppose that a male donor who became aroused while donating blood would be at a significantly higher risk of passing out or other short-term complications of blood loss.
As for my experiences:
First, you have to read over some forms with information about West Nile Virus, AIDS, and other containdicating factors. Then, there’s another form to fill out, with yes/no questions about the various risk factors. You used to fill this out yourself, but the past couple of times, a nurse has asked me the questions (in private) and filled out the form herself. They then take your temperature, blood pressure, and pulse, and prick your finger for a drop of blood to test your iron levels. The iron test may consist of dropping the blood into a vial of liquid (if it sinks, it’s enough) or centrifuging it. If your temperature, pressure, pulse, and iron levels are all healthy, the nurse gives you another paper with two peel-off bar code stickers on it, and leaves the room while you put one of them on your form and throw the other away. One sticker says “use my blood” and the other says “don’t use my blood”; after they’re removed from the backing, a human can’t tell the difference. This is in case you have some reason for your blood not to be used (for instance, if you suspect you might be infected with AIDS), but don’t want to admit it openly.
You then carry your paper over to another area and wait to be called. They sit you down in a comfortable chair (dangit, why can’t you buy lounge chairs that nice?), swab off one of your arms with iodine, and mark the vein with permanent marker. They squeeze a blood-pressure gauge on the upper arm to make the vein stand out, and stick in the needle. It hurts a little, of course, but only as it’s going in. You squeeze a little rubber ball every few seconds for 5-10 minutes and fill the bag. Then something beeps, and they draw off enough more to fill a handful of vials for tests. They press a bit of gauze against the hole and pull out the needle, then have you raise your arm for a bit and wrap a bandage around it. Then they send you off to the cantina, where you get your free cookies and juice and occasionally other goodies (sometimes local businesses will donate gift certificates, for instance).
Afterwards, you really will feel a little drained and perhaps a touch lightheaded, and you’re not recommended to operate heavy machinery. Drinking a lot of non-caffeinated, non-alcoholic fluids before and afterwards helps with this, and red meat or other iron sources is also a good idea. After about 5 hours, you can take off the big bulky bandage and replace it with a Band-aid, and by the next day or two, you should be feeling back up to normal.