Men like to sit with their legs apart, as a quick glance at a train—or indeed anywhere with seats—will attest. Sometimes, women don’t understand this behaviour; occasionally, they think that it is unreasonable and selfish.
It turns out, however, that there’s a very good evolutionary reason for it, above and beyond the normal explanation of comfort:
Scientists have known for years that an increase of even 1 degree Celsius in testicular or scrotal temperature can decrease the production of healthy sperm by as much as 40 percent...
29 volunteers, aged 21 to 35, were also asked to sit with their thighs together for an hour without a laptop. This resulted in a median increase in scrotal temperature of 2.1 degrees...
The article discusses the reproductive impact of using a laptop computer on your lap, but it turns out that it is the act of keeping one’s legs together that is actually most damaging.
Incidentally, there are other good reasons why computer manufacturers market them as notebook and not laptop computers.