There is nothing wrong with living and sleeping together before marriage
The situation at hand concerns an elderly couple that is living together and engaging in a sexual relationship without having gotten married first. At its base, the situation highlights two moral issues deeply rooted in religion: that living together and having sexual relations out of wedlock is immoral and wrong. Their reasons for engaging in this behavior are irrelevant, because there are only two sides to this issue: either you hold onto religious moral values or you recognize the illogicality of those morals.
To understand these morals, we need to consider history. These morals have been held for a very long time, since back when marriage had a completely different definition than it does today. Prior to the 19th century, love had nothing to do with marriage. Rather, marriage was an organized activity and was often seen as a sort of transaction where each party received something such as wealth, land, or lineage (National Public Radio, 2018). But if marriage was always arranged, then why have morals forbidding activities outside of marriage?
Consider the story of Cleopatra and Julius Caesar, who has a child out of wedlock for that child to hold claim to rule over both empires (National Public Radio, 2018). To prevent situations like this in everyday life, people needed rules to prevent exactly that situation from happening. So, it was considered immoral to have children out of wedlock – only an arranged marriage could produce children and lineage would continue as expected and deemed fit. Living together out of wedlock could lead to those sexual relations, so that needed to stop too.
Fast forward to today, though, and that entire idea is gone. Commonly credited to Jane Austen, marriage eventually gave way to being about the love between two people rather than an arrangement between two families (National Public Radio, 2018). With individuals now in control over their futures rather than their relatives, those individuals also need the ability to evaluate potential partners on their own terms. While you can glean a lot about someone from repeated encounters and spending time together, living with someone reveals exponentially more information. It would be unfair to demand that a couple be married before they can live together and learn the finer details of their behavior and lifestyle.
Nowadays, arguments against extramarital activities come in all sorts of odd shapes and sizes. Some spread the idea that it leads to social diseases such as AIDS, albeit without any evidence to back up the claim and plenty of evidence showing it is simply untrue (Thiroux & Krasemann, 2016). Others might claim that it encourages short-term relationships rather than long-lasting ones (Thiroux & Krasemann, 2016). But not only is that a fallacious argument, considering that sex-only relationships can last just as long as a marriage, there is also not anything wrong with a person desiring short-term relationships.
When the rules were created, marriages weren't that long to begin with. To compare a life-long relationship of the past to a life-long relationship of today is comparing apples to oranges – they just aren't the same. Prior to 1800, the global life expectancy was only about 29 years (Roser, 2019). But as of 2019, the United States is approaching the point of tripling that, with an average life expectancy near 80 years (Roser, 2019). That's an additional 70 years of lifespan that two people must spend together, and a much stronger commitment that two people are making by saying "until death do us part" than back when marriage was conceived. A short-term relationship in today's age could still be longer than a life-long relationship only a couple hundred years ago.
So it does not make any sense to consider the actions of this elderly couple to be immoral. Those moral rules only exist today as religious artifacts that do not apply to how marriage is viewed in today's world yet are still dearly held by parts of the population. As marriage evolved, as science evolved, and as our lifespans extended dramatically, the morals forbidding extramarital activities have remained exactly the same. We're trying to apply rules to a definition of marriage that no longer exists, and hasn't existed for a long time, so we're just applying it to the next closest thing instead.
National Public Radio (Producer). (2018, February 12). When Did Marriage Become So Hard? [Interview transcript]. Retrieved from https://www.npr.org/transcripts/584531641. Roser, M. (2019, October). Life Expectancy. Our World in Data. Retrieved from https://ourworldindata.org/life-expectancy. Thiroux, J. P., & Krasemann, K. W. (2016). Ethics (11th ed.). [MBS Direct]. Retrieved from https://mbsdirect.vitalsource.com/#/books/9780134010212/.