Did you know that pornography helps drive pay-per-view television (precursor to what is now Astro in Malaysia) and the VCR (the great grand-daddy of DVDs and Blu-Ray disks for you millenium kids)? Not to mention the infrastructure of the Internet.
I didn't know either.
On the scientific front, the reason for females to be unfaithful has been quantified mathematically (in a Royal Society B paper, no less).
It is beautiful, see?
For men to be unfaithful, apparently mathematics is not required (either that or scientists haven't bothered analysing such a common and unremarkable phenomenon).