Porn is a major driver of technology -- always has been, always will. Ever watch "Boogie Nights"? The porn industry is the reason we used VHS. The BBC estimates that 14% of all Internet searches are for porn.
Therefore it should come as no surprise that someone took a very user-friendly software medium for gathering interests into one central location and organizational structure and applied it to porn.
Thus, Pinterest becomes PinSex. Interestingly (and creepily), you can find other people with similar interests and reach out to them.
H/T: BroBible, Top Photo Courtesy: Brobible