A guy walks by himself he might get a couple of looks but put a girl next to him (especially if she's pretty) and all of a sudden it seems every girl stares. what's up with that?
Most Helpful Girl
... Honestly, we could just like her shoes.
But, turn it around. A girl of average looks - not ugly, but nothing to give yourself whiplash over, either - walks by alone. You don't notice. But, the same girl has a guy, particularly an attractive guy? That means that there is definitive personality aspects to a physically common person that makes them more attractive than their peers.
Having someone of the opposite gender is an immediate sign that there is something worth looking at. It's not always true, but it's also not wrong.
It's the same as hiring employees. Companies are more likely to hire people who have jobs because it proves they're hire-able.