@tendertouchThe 'forbidden love' is more akin to Romeo & Juliet, but guilt or opprobrium isn't a requirement, as 'forbidden' love can also include ANY unusual pairing, whether it's gay, interracial, a mixed-class or a mixed-background story, no matter how it pays out. Again, tropes are simply proven, well-established story concepts, so any author can put their own spin on them. Thus, as simple story concepts, there's NO copyrights involved.
These are all literary definitions, so I pay more attention to the more literary distinctions, as that's long been my particular focus.
As far as 'three-legged' stools, that was a long-time analogy, which was never very appropriate, as it was equally used to support and attack those stories. So, consider the well-worn milking stool analogy, where after years of heavy use, they are more likely to topple compared to a more stable bench (i.e. if a single leg breaks, the other three will generally continue to stabilize it).
Again, I didn't invent the ancient analogy, just referencing it.
Yet once again, I've beat yet another discussion thread to death, so ...