punch below (one's) weight
1. To perform, achieve, or do something at a level that is considered to be lower than one's abilities, talents, or personal attributes. The Oscar-winning actor is truly punching below his weight in this new film, with a performance that is dull at best and downright amateurish at worst.
2. To be romantically or sexually involved with someone who is considered by others to be inferior in physical appearance, intelligence, or success. It's odd to me that someone so motivated by his career would punch below his weight with some gas station attendant like her.