I've only read Tokyo Babylon, not X, so I didn't see the denouement, but I think Subaru/Seishirou can be interpreted as a tragic romance? Tragic in the sense that the potential for love ran afoul of the natures of those who wanted it, not tragic in the sense that the love was misunderstood by society. There was an incredibly powerful connection between them, even if it was deliberately orchestrated on Seishirou's part, and there was something on both sides, some genuine emotion or attempt at emotion. The problem was that Seishirou was an honest-to-someone sociopath who genuinely did not seem to grasp how love worked, and Subaru was fragile and empathic enough to near-destroy himself trying to find something in Seishirou that would translate to how he understood the world, when such a thing honestly didn't exist. The tragedy was that they went looking for something in each other that one of them didn't understand and the other one couldn't give, and spectacularly destroyed themselves/each other trying to find it.

I mean, I think you could call that a tragic romance? In that it has a twisted attempt to love each other (based on extremely suspect definitions of 'love', admittedly), and pretty much all the tragedy you could ask for?