You've probably seen a few couples that made you go.. huh? How did that guy get her?

"If you see a good looking woman with a man who is not all that attractive then you've got to stop and think, what is it that this guy has? He must have something," said Melody Brooke, a licensed marriage and family therapist.

A new study published in the Public Library of Science had 30 men and 30 women look at pictures of couples. They were asked to rate how desirable they'd find long-term relationships with the opposite sex. Guess what? they liked the ones with attractive partners. The study also found that men are less discriminating.

"She just has to be hot to me. If she's attractive to me I really don't care," said Thomas Kelly.

Brooke says that's because biologically women look for qualities of a good provider, while men look for a mate to carry on their genes.

Many women like to do research. For some, that means Facebook, where you can check out pictures of guys and their girls.

"They become more attractive. These girls want me.I hang out with attractive girls, they find me desirable, so it's just natural that you would start to think that. It's just good marketing," said Michelle Deloy, of Dallas.

"You want to see what their interests are and you want to see what kind of people they've had in their life, what kind of women they've had in their life and it gives you better information about whether this is somebody that would make a good mate," Brooke said.

Brooke's advice: be careful, because appearances can be deceiving.

"Don't use that as the entire criteria, but just use it as more information," she said.