Comedy, Satire, Dystopian Literature
When we say that The Wanting Seed is a comedy, we don't mean like funny, ha ha; instead, we're thinking of the conventions of comedies like William Shakespeare's Twelfth Night and Much Ado About Nothing, where heroes and heroines have to face trials and tribulations (sometimes hard, sometimes hilarious) before they finally get to say "I Do" and smooch.
That said, although The Wanting Seed draws on comedic conventions, it also treats them satirically, which is why we're never quite sure if Beatrice-Joanna and Tristram's marriage is a good thing. Although the end of novel seems to validate their union—and their bubbling baby twins—we also know that they're now living in a world where nuclear families are celebrated because they make more babies for the State (and thus, eventually, more bodies for the British Army and the canning factories).
The fact that Beatrice-Joanna and Tristram's diet now consists entirely of tender juicy man-or-lady-flesh is a pretty strong clue that the novel's ending isn't as rosy as it seems. Ultimately, husband and wife are still living in a dystopian England, where State repression has given way to a more subtle—but no less deadly—State tyranny.