True. However, this doesn't change the fact that there is still a definite biological and evolutionary logic as to why so many people turn towards activities like sex, drugs, and food for gratification of the "self," as opposed to say, staring into the sun for hours on end, or sticking their hands into meat grinders.
Your body wants to engage in certain behaviors, as they serve a certain purpose. It rewards you for them as such.
Likewise, it punishes you for other behaviors which might bring about obvious harm.
It simply happens to be the case that the system isn't foolproof, and really wasn't adapted to deal with today's world. For that reason, things can often have a tendency to get confused.
A caveman simply wouldn't have had enough food on hand to become obese, for instance. Today, a great many people do, and they wind up losing control of themselves because of it.
Much the same is true of sex. It is much more widely available today than in our evolutionary past, and seemingly available without consequence (of the immediate variety, anyway).
Just like with food, a lot of people lose control of themselves and wind up getting in trouble for that exact reason.