The two themes of Nadine Gordimer's "Town and Country Lovers" intertwine. The themes of interracial love and consequences are utterly dependant upon one another. Since interracial love is forbidden, and Dr. von Leinsdorf, the cashier, Paulus, and Thebedi do not care, they are all subject to the consequences for breaking the law (regarding interracial relationships).
Without one theme, the other theme would not rise above the simple story provided by the text (I am not using "simple" in a derogatory way here--only suggesting that a theme is what adds depth to a text).
That said, the theme of interracial love goes deeper than just the act of one race falling in love with another. The text examines the reasoning behind the relationships. One out of convenience and one out of habit, both relationships posses an imbalance which mirrors society (that of the white males having power over the black women).
The theme of consequence illustrates (since the story takes place in both the town and the country) that no one is "safe" from law. Regardless of where one lives, he or she is still held to the laws which bind them to their society. Although love is not something which should be governed by law, the novel depicts it as something just like theft or murder--it is subject to punishment for breaking it.