marry above (one's) station
To marry someone who is of a higher social class or standing than oneself. A: "I hear that the local fishmonger's daughter is betrothed to a rich foreign lawyer!" B: "My word, she's certainly marrying above her station, isn't she?" For all the talk that social classes have been wiped away in recent years, you will still find people who believe one can't or shouldn't marry above one's station.