Factual error: The mall that the plastics state they are going to, Old Orchard as stated by Cady, is an outside mall. However, the mall they go to is an indoor mall.
Factual error: While the plastics are at the mall there is a Bell Mobility booth seen behind them. This booth wouldn't be found in an American mall as they only service Canada.
Answer: Precisely because he was sleeping with a student. Getting her pregnant would bring their affair out into the open and create a scandal for the whole nation to know what he was up to. Therefore safe sex was really important to him.
The_Iceman
True, but since he was spending time with her, he could make sure that birth control was being used (i.e. him using a condom). I think he was trying to appear "uptight" about sex, so he could be the least likely suspect.