Genital stimulation reduces sensitivity to pain but not to sensations of pleasant touch as the sex act releases naturally produced narcotics in the human body.
The body compensates for the presence of narcotics whether natural or artificial so that, when those chemicals are not present, withdrawal sets in. So, after experiencing sex a number of times, it becomes physiologically painful to abstain.
Physical withdrawal, emotional dependence, peer pressure, sexual enticements, pubertal stress, curiosity, genital stimulation, kissing and cuddling combine to produce a physio-psychological experience awesome enough to cause humans to mistakenly conclude that they can not resist an "inborn sex drive".