The Danger of Secrecy
Despite his friend Jack's urging to the contrary, Thomas engages in a secret relationship with Harriot. The couple fears the judgement of their community. Perhaps they rushed into the relationship. Regardless of the reason, though, the secrecy of their affair breeds shame between them. When the community learns of their relationship, they respond with horror because they simultaneously learn the news that Thomas and Harriot are actually siblings. If the couple had been forthright with the people who cared about them, they could have spared their reputations. As it happened, people assumed the two knew that they were engaging in incest and decided to proceed secretly. Additionally, Thomas and Harriot could have benefitted from the support of their community. Maybe they would have lasted longer.
Subversion of Intimacy
The characters who are supposed to be close to one another and informed of one another's lives in this story are not. In fact those who have no business becoming intimate with one another do, at the price of their reputations and even sanity. For example, as Thomas' sister, Myra should be reasonably close to him. She, however, is one of the last to learn that Thomas is seeing Harriot. He spends all of his time in private, with Harriot, developing a romantic relationship with this woman who is actually his sister. As for Jack and Thomas, they are close to one another at the beginning of the narrative, but with time they drift farther and farther apart as Thomas and Harriot's secret starts to weigh upon Jack and to place immense distance between the two of them. The protections and benefits of these relationships which naturally exist fail to continue as the characters all shift into subversive forms of intimacy which compromise their pre-existing relationships.
The Weight of Reputation
Essentially the protagonists -- Thomas and Harriot -- both kill themselves out of shame. They are driven crazy by the reception of their relationship by their community. Dreading this response, Thomas chose to conduct their relationship in secrecy, despite the warnings of his friends. Despite these precautions, the truth comes out eventually, along with the discovery that Thomas and Harriot are also siblings, which is news to the couple as well. Devastated by the knowledge of their incest and the inevitable end of their relationship, the two lovers are isolated from the rest of their community. They are left to individually navigate the wake of their relationship and the loss of innocence. Unable to cope with these enormous pressures, they each independently commit suicide out of despair for their lost social standings and the confusion of the sordid intimacy which they had experienced with one another.
