Generally I would agree with this post however I differ on:
Part of the reason the left arm is preferred is because it is closer to the heart; also, it is part of standardization. If you are wanting to see changes in your BP over time, then you want to measure it the same way (same time of day as well) each time.
One reason to look at BP in both arms is to look for large differences that might be indicative of some sort of circulatory issue.
Within a minute or so, the restrictive forces in re-taking a BP would be gone and you would could get a similar reading.
The real concern with most BP units of the manual type are calibration and operator error. The calibration can be way, way off and it is really impossible to tell without checking it against some standard.
I disagree that at rest a trained person's systolic pressure would be higher. This really goes against a reason for people to exercise (that is to lower BP). In fact, a trained person might have a lower systolic because the blood vessels are not going to be as restrictive to the flow of the blood.
I am not sure how many BP measures 123 has done, but systolic pressure during exercise can be very, very high. At a high HR, the venous return is still very high. While the LV might not be filling completely the rate is such that the time between beats is very small so the pressure remains high and near constant. Diastolic pressure will decrease or stay the same (rarely rise) because of vasodilation from other factors at work on the smooth muscle of the vessels.
Some people do look more closely at diastolic pressure in terms of CV disease for some of the reason you note. Also a t rest (which is where we spend 22+ hours of the day) your diastolic pressure is the pressure exerted on the artery walls 2/3 of the time. In some ways that is more strain than the occasional pulse of high pressure blood coming by.
As for the penis issue, blood flow is restricted to the penis most of the time. Not really sure that this has to do with the issue at hand other than some BP meds create erectile dysfunction and it seems like many of the posters here think with their dicks.