At the beginning of “To Room Nineteen” Susan and Matthew are intelligent, cautious people. They are very loyal, honest, and careful people. When Lessing mentions “their abstinence from painful experience,” she is highlighting the results of this rational, careful behavior – by conducting themselves moderately at all times, they do not become involved in any experiences that could cause themselves, or others, suffering. They eschew complicated scenarios and live their lives according to the most logical, intelligent path, which makes them reliable and sought-after for advice from their friends. The fact that they have both had several partners and are marrying late (compared to all their peers) is a testament to this. Indeed, when they do finally marry, Lessing states that “because of their foresight and their sense for what was probable, nothing was a surprise to them.”
Both of them are moderately successful, unimposing members of society who value balance and compromise, so much so that they were attracted to these self-same qualities in each other, “linking themselves ' whom no one else had ever thought of linking, probably because of their similarities.” And when they developed into a romantic couple they believed it was evidence of their “sensible discrimination.” This “sensible discrimination” could be considered another form of their “abstinence from painful experience” – they do not take risks, and have come together through the identification of this quality in the other person. They are a safe match, and believe themselves to be as such.