[@Rakurai]

Hmm... you convince me in strange ways. Actually Claudia will milk the fun out of it, but marriages between humans and Spirits have been reported, mostly between an Spirit and its respective Knight, like how people can marry body pillows and visual novel characters on those weird places known as Korea and Japan. Outside relationships aren't that common though. Either way muggles and even other Song Knights/Spirits tend to frown at such relationships.

Even then, there's stories about half-Spirits, in the legends, though none of them has ever been confirmed to be true.