We see Mary at the kitchen table of her friends and as the conversation passes her by she realizes she doesn't matter to them any more. Worse, we see it dawning on her that she may never have mattered to them at all in their relationship of 20 years. The comparison with her car is inevitable, a vehicle that should have brought her a sense of freedom, but turned out to be a lemon if ever there was one. As another commenter wrote, it is a scene of exceptional cruelty. (Can I nominate Ms Manville for an Oscar, please?)
How could this insincerity, this not-so-mild form of wickedness persist over such a long time ? Perhaps it has something to do with the British fondness of manners, but I think it has to do with work, more specifically in Tom and Gerri's case with having a profession, being a professional.
A professional is someone who is applying a set of rules, an algorithm to a standard input to produce a standard output. (What distinguishes a professional from a craftsman is that the input, rules and output are so complex that they defy supervision.) Tom and Gerri are professionals and what they are doing is extending their professional attitude to their personal lives. Their relationship with Mary is professional. Their marriage is dealt with professionally. Their marital bliss is ultimately based on their contentment with the standardized output it produces. Tomatoes anyone ?
Contrast this with Ken and Mary. Ken is an alcoholic in bad physical shape. He was once handsome however and has a good heart. His problem is that he questions the meaning of his job, which in his case seems to amount to questioning the meaning of his life. And then there is Mary. Her goal in life is Love, not work, thereby committing the ultimate sin (not making work your life goal that is).
Two scenes illustrate the stranglehold work has on our lives (and the importance of it for the movie's theme, I think). In the first an Asian couple is visiting Joe, an old man threatened with eviction and a young woman acting as his interpreter, her ability to help the old man, however, limited to the duration of her lunch break. In the second Carl arrives too late for his mother's funeral, having been stuck in a traffic jam. The funeral couldn't be postponed however, another one was already waiting.
So are Tom and Gerri right ? At the dinner table Tom is telling of how he and Gerri met, by chance on their first day in university. This detail hints at the internal inconsistency of their way of life. And of course right at the beginning of the movie there is this session of Gerri with the sleepless patient, an Everywoman. It shows her utterly failing in her role of counselor, her very profession.
Mary is indeed looking for love in all the wrong places. At the end of the film you realize that, despite all appearances Tom & Gerri's was such a place.