On reading this title, you spontaneously completed it with ‘…men think about sex’. True or false? Yet the words ‘Every three seconds’ tell us nothing about sex, but a lot about maths – because three seconds is a measurement, and measuring belongs to the queen of sciences. This statement invites us to give more thought to sex and maths. If we define maths as the search for patterns and structures, and if you assume that all human activities display patterns, then it must also be possible to take a mathematical view of our sex life. This evening, Van Bendegem brings such patterns together. What does the reproduction of rabbits have to do with the golden section? Is there a strategy for finding the ideal partner? What are the mathematical limitations of dating sites? How can the complex dynamics of a relationship be defined? How do STDs spread? And why does more sex guarantee safer sex?