Women with steady partners may still be tempted to sleep around - but mainly on certain days of the month, say researchers. A BBC documentary to be broadcast on Wednesday explains how human sexual instincts are so strong that some women's preferences may alter significantly while they are ovulating. While her partner might be a better bet to bring up children and support her, another man might carry genes which mean healthier, stronger children. Morgan Wise, a train driver from Big Spring in Texas, found this out when his youngest son was found to have cystic fibrosis, a devastating lung...