The Best Toronto Campus for Fucking award goes to U of T's St. George campus, where you'll find plenty of unused areas in various buildings, almost guaranteeing you a place to go at it undetected.
An obvious choice is the upper levels of Robarts Library, where you can easily access private study rooms (put a sheet of paper over the door's window, dummy) and weird little nooks and crannies.
You may wanna try something a little less risky -- perhaps one of the basement rooms at University College, which sit empty for long periods of time. Or try UC's popular student hangout and study area, the JCR. If you're creative and willing to wait for less busy times (before 10 am or after 5 pm is usually a safe bet), head upstairs to the balcony before you go "downtown."