Run Papa Run(2008)

A lawyer falls in love with a gang boss, gets knocked up and drags him to the altar. One evening, cradling the crying infant, he metamorphoses into a doting daddy, and henceforth devotes his life to keeping her from finding out his true identity. His light and joy grows into a teenager, and alas, falls for the son of a cop, while rival gangs move in on his turf.