Despite what some paranormals might believe, most hybrid paras are not the result of humans being easy, degenerate sex fiends, or hellbent on removing paras from existence by outbreeding them and replacing them with more humanlike offspring. Rather, it is more of a coincidence, inasmuch as such things can be.
The truth is that mankind is more pliable, in this regard, than practically any paranormal species, leaving aside those specialised in picking up the traits of other species. In most cases, mages and psychics are human enough (leaving aside those who have altered themselves to inhuman levels, or become liches) that the only difference between them and mundanes having children is taht the child will have a chance of being magical or psychic.
Most paras' genetics and metaphysical equivalents are resistant enough that if, for example, a dragon were to have children with a Fae, they would either be one or the other. On the other hand, a human having children with a Fae results in demifae, who often share their Fae parent's powers but not the weakness to iron (or, if they do share the latter, it is diminished, leaving painful but healable wounds). However, since demifae usually lack the hyper focused minds of their Fae parents, they prefer to stay away from Fairie, where they would be seen as insane.
The most famous type of human/paranormal hybrids are likely demigods, who are born from the union of humans and deities. Demigods vary wildly in terms of abilities, but usually inherit a fraction of their divine parent's power. Even so, some are effectively mundanes with uncanny luck or insight, not even exceptionally long-lived. The descendants of demigods, sometimes referred to as godlings (though a certain American writer of children's literature, beloved by struggling paras and their parents for his series' focus on young paranormals integrating into mundane society, has suggested the term legacies), are usually weaker than their parents, though the amount of ichor in a demigod's veins is not always an indicator of power. Many of these children pick up other abilities by chance, to the point their diminished divinity is a facet of their powerset. It is debatable at which point a demigod's descendant lacks enough ichor to be considered divine, as this seems to vary by person.
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Some paranormal species have little to no issue interbreeding with humans, Fae and dragons being the most famous examples. Others can have children with humans, but they resemble one of the parents rather than being hybrids. Zmei, iele and the majority of fertile paranormals are in this category. Weres technically are, but, as the children of weres are born humans when they don't inherit the other parent's traits, they have human children have the time, regardless of their partners' nature.
Undead are unable to have children. People who say this could easily be changed through faithcraft often hear counterarguments that the bareness brought by undeath is the result of a divine curse, cast for reasons varying from punishment to the safety of the world. In any case, the pantheons have been closemouthed on this subject. The analysis of future possible timelines and alternate realities has discovered the existence of human/undead hybrids, with dhampirs being the most numerous. Dhampirs are half-human, half-vampire, sharing their vampiric parent's power, though not their thirst for blood or the weakness to sunlight, as they can use their esoteric abilities in broad daylight with no issue. Also, while they do not need to drink blood, doing so makes them more powerful, like it would a vampire. The weakness to holy power remains, with some saying this is only natural, as the fruits of such unholy unions do not deserve to exist.
Recent events, however, point to DEATH's Keeper planning to find a way around the undead's inability to reproduce. David Silva's comments indicate that this research began for personal reasons, though, even if he did not want to help other undead have children to, he could not prevent it, as the changing of this fact of existence would affect all of them.