What is important is that the two partners are relaxed, enjoy what they are doing, and are in relative agreement regarding the frequency of sex. At the extreme there is a lack of desire. The man no longer thinks about or wants to have sex. Unfortunately, the absence of desire and the absence of erections means that the penis is not receiving the nutrients and oxygen it needs to remain healthy and functional. In this situation, muscle in the penis, that is the erectile tissue, is increasingly replaced by fibrous tissue.
Fibrous tissue is less elastic.
As this occurs, it becomes more difficult to get an erection. A vicious cycle is created. The more failures a man has in getting an erection, the less he wants to try to have an erection. Finally, he gives up and loses his desire. With Alprostadil we have seen how the vicious cycle can be decisively and quickly broken.