A 38-year-old man caught masturbating near a bridge in Raritan Borough last week is also responsible for a pair of sexual assaults in parks in Somerset County in the past year, authorities said Wednesday. Rogelio Calyeca Postrero, of Raritan, is charged with two counts of aggravated sexual assault, lewdness and knowingly exhibiting false government documents, the Somerset County Prosecutor’s Office said. Investigators said they linked Postrero to the sexual assaults after arresting him on lewdness charges Thursday in the area of Nevius Street Bridge in Raritan Borough shortly after 9 a.m. Afte…