The mechanisms in the penis that keep it flaccid are increased considerably when you are active and performing physical tasks.
Our body goes into a more parasympathetic state while digesting food "rest and digest".
The active sympathetic state is reduced so as to favor blood flow to the digestive organs instead of muscle tissue. A similar thing will be occurring in your sexual organs, stimulation from NE will be slightly lessened in these moments causing relaxation of the smooth muscle in the penis making it appear larger/relaxed. The same thing occurs in the morning when you first wake as sympathetic action in the body is at its lowest, until you get up and start doing physical activity.
Testosterone has been shown to modulate the expression of genes (either directly or indirectly) that alter the expression of alpha-1 adrenergic receptors. That is it may increase this expression of these genes in various tissues if testosterone is in excess. The implication of this is; if it was to occur in the erectile tissues, they may be far more responsive to NE stimulation and sympathetic activity in the rest of the body. The net effect would be a more shriveled looking penis most of our waking hours and erections that are more difficult to achieve and or maintain even when sexual stimulation is initiated. This is something I have been researching for awhile now as I have suspected that TRT may cause issues in the autonomic nervous system in some men, mostly concerning testosterone levels that are in excess of that which is encoded in our genes. It may be that it doesn't have to be in excess by very much at all for these issues to occur over time, gradually.
Guys who are the more nervous and anxious types, may be much more susceptible to this effect than others.
When I first wake in the morning I have either a partial or full erection most days. If I do not, I am able to get one with great ease whilst I am still in this half awake state. After I am out of bed and much more awake, this begins to change and "everything" doesn't look quite so relaxed. After I have a coffee, it all changes dramatically. My penis generally looks much tighter and is far less responsive to touch. The sudden increase of Adrenalin that coffee stimulates affects my genitals much more now than it did in my younger years. It would appear, I am now much more sensitive to the effect of NE in these tissues now.