I just came across a study reviewed in LiveScience that suggests your partner’s hotness makes you seem more attractive to the opposite sex. The results were more true of women than men, who pay attention to attractive women no matter who they’re with.
From the review:
Scientists had 30 male and 30 female volunteers who all described themselves as straight rate how attractive they found photos of 36 men and 36 women. The volunteers were then shown 144 pictures of men and women paired together and asked how desirable they would find long-term relationships with members of the opposite sex in the pictures.Both male and female volunteers rated people in the pictures as more desirable when they were paired next to attractive companions, the scientists found.
By using cameras to track eye movements during the experiments, the researchers also saw that when volunteers spent more time looking at a potential mate’s unattractive partner, they were less interested in that mate.
Female partners also said they were more skeptical of attractive men if they were with an unattractive partner.
You can find the entire study online here at the open-access journal, PLosONE.