Underworld Orgy

Question: How do you justify death gods having biological kids?

...the same way you justify any god having biological kids?

Death gods may wield power over life and death and hang out in dreary, glum underworlds a lot of the time, but they still have reproductive organs and they very obviously use them in many of their myths. In fact, most of them have children of their own in mythology already, so they're clearly not hampered by infertility. Here's a quick list of the death gods we currently have in play in Scion, for comparison:

Gods with Death who have children: Anubis, Ereshkigal, Freya, Hades, Huitzilopochtli, Indra, Manannan mac Lir, Morena, The Morrigan, Nephthys, Nergal, Odin, Persephone, Shiva, Tlaloc, Veles

Gods with Death who don't have children: Hecate, Hel, Kali, Mictlantecuhtli, Yama, Yanluo

Gods with Death who are incapable of children: Izanami, Osiris

So you can see that there are almost three times as many death gods with children as those who don't have them, and only two we know for certain can't have any (Scion addresses the case of Osiris, in particular, noting that he can only adopt Scions, not create new ones himself; they don't really talk about Izanami, but since her vow is to destroy life while her ex-husband creates it, I would assume she probably refuses even if she's capable). Like gods of all other things, death gods were considered by their cultures to be powerful, important beings who ruled over a facet of the universe, and that usually translates to them having families and children, either as gods of related concepts who help them or as general shorthand to remind you that they're important, since most ancient cultures would have considered a childless couple less than badass since they'd be incapable of carrying on their awesomeness through subsequent generations.

Of course, there are more symbolic concerns when it comes to death gods having children, as well as logistical ones. Some death gods are ambiguous when it comes to whether or not they're "alive"; people like Osiris, Izanami and Yama all died at some point in their myths, which would make it hard to reproduce, but at the same time they're obviously still "alive" enough to be important powers in their own right, making it clear that they're not just ghosts drifting around in the underworld. Individual STs will have to decide what their deaths really meant - whether they didn't really die and the phrase is just symbolic of their descent to the underworld, whether they did die but have since been resurrected in some form (indeed, in the case of Yama he's at least alive enough that he can get killed again when he annoys Shiva), or whether they died but thanks to The Reaper still get to reign over their domains (works for some, but for many death gods, they wouldn't gain that association until after death anyway - I doubt Izanami had The Reaper when she was busy being a creator goddess).

There's also the question of death gods that are confined to their realms; it's not all of them and it's not all the time, but in particular, Hel, Izanami and Osiris seem to be stuck down there, and there are others, like Mictlantecuhtli, who we don't see directly constrained to stay there but who also never seem to leave. At that point, it's a question of why those gods are prevented from leaving the underworld and whether there's a way they could get around it; for example, it's implied that Hel can't leave the underworld because Odin won't let her, not because she'd be otherwise incapable, and the Scion books imply that to help the war effort he allows her out on sort of parole in order to make Scions. If the god just doesn't seem to ever leave but there's nothing that tells us they can't, I'd assume that they can throw up an Avatar and go to the World if they want to just like every other god, though with the heavy focus on responsibility that a lot of death gods have, they probably don't do it all that often.

There are your occasional death gods that very directly represent infertility and inability to reproduce; these would include Izanami, Nephthys (who, despite being Anubis' mother in early versions of the myth of his birth, is still cursed with the depressing epithet she who has no vagina) or the Canaanite Mot, who are so directly representative of death that it is impossible for them to create life. There are very few of them, and I'd suggest treating them just like virgin goddesses or strictly monogamous gods: they probably adopt their Scions.

Everybody's pitching in to deal with this Titan problem, and death gods aren't the only ones who usually don't go out into the world and have sex with random mortals; them getting up to do so in an effort to save the world is no weirder than folks like Amaterasu or Ogma or Ra randomly nipping down to the world of humanity for a quick shag. Children of death gods are not only possible but actually crop up all over mythology anyway; there's no reason to assume that being the lord of an underworld suddenly makes your divine balls shrivel off, which is amply demonstrated by the many minor gods and heroes who call a death god father or mother.

Like most questions of parentage in Scion, look at a specific case and see if there are any circumstances that require special handling (i.e., sworn virgin, castrated, locked in the basement); if there aren't, there's no reason not to let a Scion come into being the old-fashioned way.