you and your partner have established a rule that says 'don't look at other people', or if this is done while your partner is describing something about their experience or vulnerability.
So why do we do it?
Humans are animals, and as animals our priority is to procreate, so evolution wants both men and women to seek potential partners which will maximise the chances of their survival as a race.
A recent study in Evolutionary Psychologyand a 2013 study, found that women first notice a man's upper body (because they're wired to find strength), whereas men notice women's breasts, waist and hips.
The studies used eye-tracking software to map the visual activity of both men and women with different body types. Men and women both had more positive responses to women with "curves", which the study concludes is due to men being hardwired to prefer curves, as they signal fertility.
Researchers believe men have the highest chance of continuing their line by mating with as many partners as possible, which is why men are more likely to look at women after they've become sexual in monogamous relationship, according to Dr. Nancy Irwin, a therapist based in Los Angeles.
A 2009 study, published in the Journal of Experimental Psychology, showed that, when presented with photos of attractive people, those in relationships spent less time looking at the photos than single people.