Here's how they did it. They had people try and keep track of flashing shapes on a screen. Occasionally, all these faces would pop up in the opposite corner. Single people would slow down when any face showed up. People in love would slow down if "average-looking" faces appeared, but if it was a "highly-attractive" face they did not slow down, so basically they wanted nothing to do with the highly-attractive face.Image via Wikipedia
The researchers concluded that, from an evolutionary standpoint, you are primed to find "romantic alternatives" less attractive when you are romantically involved with someone else.
But couldn't it just mean you are afraid of being tempted? Don't look. Don't look. Don't look.
Either way, it seems like this is research you can use. Thank you science.