I have what appears to be scar tissue in the form of a straight line
all the way down my penis, over my testicles and ends after that. Is this
unusual for a guy? I am circumcised. Is this scar tissue from that or some
strange webbing from birth?
From your description, the scar tissue appears to resemble a "seam" on the underside of your penis. This seam forms when a fetus is in the uterus. In women, the seam becomes the inner lips of their vagina . In men, the seam encloses the urethra along the length of the penis, and joins with the head of the penis on the underneath side. What looks like webbing to you is called the frenulum. This is perfectly normal; and, for some men, it is quite a hot spot in the pleasure arena! Remember "the butterfly flick" (in which someone gently flicks her/his tongue lightly, back and forth, across this spot)?
Wouldn't it be great if everyone knew their bodies so intimately that
they were familiar with the names of each part?