It is commonly believed that men think about sex a lot. In fact, some people go so far as to say that men think about sex once every 7 seconds. Is this claim true? What evidence does the claim depend on? Well, according to snopes.com, the claim isn't true. Snopes.com goes further to discredit one of the sources of evidence supposedly proving the claim.
So why do people believe this claim? It, apparently, has a lot to do with genetics and common assumptions about human nature. Snopes.com notes that men are generally more motivated for sexual reasons that females, but snopes.com also mentions that this condition isn't due to some conscious motivation, choice, or social learning; this sexual motivation is due to how males are physically wired -- genetics. From this knowledge, though, the data have been misconstrued to 'show' that men think about sex constantly, or at least once every 7 seconds. Without any actual data to support the claim, though, the notion doesn't withstand its own weight.
Actual experimental data demonstrate that men think about sex 19 times per day which computes to less than once per hour (not considering hours of sleep because it's impossible to say given the subjects in the study as described on snopes.com). Women aren't immune to sexual thoughts, though, as they think about sex 10 times per day. Following this, to say that men think about sex every 7 seconds would then mean that women think about sex every 14~ seconds. However, it's clearly been demonstrated that men aren't so enthralled by sex as to think about it nearly constantly. The data do not support the claim.