Addressing Mitchell's discomfort with PDAs was a response to a minor controversy related to season one episode "Airport 2010". When two couples reunite after a frantic trip to the airport, Phil and Claire kiss while Cam and Mitchell merely embrace. This led to some complaints from LGBT viewers who thought the unequal displays of affection were out of fear Of backlash from straight viewers.
In discussing Mitch's aversion to PDAs Cam says that "Gays don't high five". In fact, Glenn Burke, a gay man, is credited with inventing the high five.