Man... A guy just can't stay away from a topic like this - if only as a voyeur.
Yea, that would be YOU buddy!
Bruise, this whole thing is both mental and physical.
When you're age 15 to 25 and just swimming with more testosterone than you can possibly know what to do with, you're ejaculating in your dreams. Guys usually settle down to be the best they can be from about 25 to 40 - issues aside. From that point on, you'd better hope you have good genes, and have been taking good care of yourself (no smoking, no overeating, no issues with diabetes, no issues with hypertension meds, etc., etc.).
Meanwhile... You can have the most beautiful woman in the world beside you who meets all your "on paper" specs for being THE woman. But if chemistry isn't there, forgetaboutit. And that is soooo complex. There are lots of studies on pheromones humans pass back and forth between each other, one of which suggests that "chemistry" is a function of tissue types (the more dissimilar, the better the chemistry). Being mentally "in tune" is a big factor. Tapping into the emotional element w/o being stressed is important. Memory and association is important. It goes on and on. So just because you're shooting blanks with Sally doesn't mean Sue won't knock your socks off. And just because Jane can't get you excited doesn't mean you won't break the kitchen table with the most incredible after-an-argument sex you've ever had in your life. You just need to circulate, man!
As for the meds and physiology, well this is complex as you well know.
All Viagra does is help smooth muscle maintain its function. It was originally supposed to help perfusion in myocardial tissue, and the idea (concerning nitric oxide biochemistry) won someone a Nobel Prize. Then some bonehead (er...
) thought 'Gee, if it helps this tissue with perfusion, then maybe...
' Only a guy would have figured that out. The rest is Wall Street history. The truth of the matter is Viagra probably won't do squat for a healthy young male. This is for older men who need to maintain erections longer because their biochemistry isn't quite as young, or they have other confounding issues.
You are on to something with the sympathetic, the coffee, etc. It is complex, as you know. Along those lines, yohimbe has been the herb of choice to manipulate sympathetic. Yohimbine - the active ingredient in yohimbe - is a selective antagonist of alpha 2-adrenergic receptors. It is used in many health food products as an "aphrodisiac." The only problem is that its effect on SNS and norepinephrine release will raise your blood pressure. It also can give you a headache and tinnitus (ear ringing).
On the flip side, many blood pressure meds which manipulate these same neurohormonal responses can REALLY screw up a guy's love life. Mick Jagger got it right; 'What a drag it is getting old!
But you do what you gotta do and get on with life.
Sooo.... Why not invite a young lady out to Starbucks, and see what happens?