The Penis
Mounting sexual excitement produces an obvious change in the penis.

Tumescence:
The three spongy bodies inside the penis (the two corpora cavernosa and the corpus spongiosum) become tumescent, i.e. they are filled with blood and thus cause the penis to rise and stiffen. Therefore, the most obvious sign of sexual excitement in the male is the erection of his penis.

Insufficient or Absent Tumescence
Sometimes a man may fail to achieve or maintain an erection of the penis, although he feels excited and is eager to have sexual intercourse. Obviously, in this case he is also unable to proceed to the other phases of sexual response. Such an occasional lack of erection may have many causes, but can usually be traced to particular circumstances in a specific situation. Both sexual partners should accept the incident with equanimity and perhaps turn to forms of lovemaking that do not require an erect penis. There is no cause for concern. However, if the same problem should occur frequently or even regularly, professional help may be advisable.