For a poem about love, "Living in Sin" is kind of a downer. Love is treated in terms of the contrast between idealistic ideas and realistic relationships. We don't really know what went wrong in this relationship, but we do learn that, like homes, relationships don't just keep themselves up. They take work. And if you're not willing to put in the work, the relationship will just continue to deteriorate.
Give it the old college try, guys! The poem suggests that relationships require effort, and if they had simply put more effort into it, the relationship might not have gotten to the state of disrepair that it is in now.
Nah. Why bother? Relationships do require a certain amount of effort, but the feeling of love between two people is what causes a desire to put in that effort. If that feeling isn't there, no amount of effort can keep the relationship from deteriorating.