a bird in the hand is worth two in the bush
Sometimes you're just too busy staring at your own cock to notice the other guy's.
He was so busy looking at his own cock, he didn't even notice the other guy's cock was bigger.
She was too distracted by the guy she was with to see the guy standing behind her.
He was so focused on his own cock, he didn't realize the other guy had already taken the chick.