Also known as romance, the love genre has a core value of love vs. hate, and the core event in the love genre is proof of love for one character to another. I go over the romance plot structure in quite a bit more detail here, but in The Story Grid Shawn Coyne does introduce a few subplots within romance.
Courtship: Two people meet and either commit to one another or not. In addition to love vs. hate there tends to be some focus on independence/dependence, communication and misunderstanding, morality/immorality, togetherness/loneliness, and the fan favorite, societal approval/disapproval.
Marriage: Not simply a matter of ending with wedding bells, these stories go deeper into already committed relationships. These stories tend to deal in fidelity vs infidelity, truth and lies, betrayal, and valuing of the self over the family. Friday Night Lights does a great marriage subplot.
Obsession: Obsession stories tend to stray into horror. They go beyond courtship and tend to focus on desire, fear, and survival.
Further subgenera include ….
Erotica, which I’ve heard best described as a matter of lighting (i.e some romances fade to black, spicier romances may describe feelings and such but the “camera” (reader perspective) is mostly pointed at the face, and erotica is fully lit descriptions.
Gothic Romances, which also tend to delve into horror. Riverdale played with this subplot a good bit.
Regency, suspense, western, crime, thriller, horror, you name it.
Romance is a stand alone genre and it can pull any major genre in as a sub genre (like J.D Robb’s In Death series), but it is also the go-to sub plot. It’s difficult, incredibly difficult, to find a story that doesn’t feature a romance in some way, shape, or form.