A compelling moral argument could be made in the case where they have human sexual partners; I would not consider having sex with road kill to be especially safe from a communicable disease standpoint, and engaging in this behavior endangers the partner’s life.

I’m willing to bet the partner doesn’t know, too.

That said, pretty sure the whole spilling seed things comes from a pretty blatant misunderstanding of genesis 38:8–10. It’s part of the story of Judah and Tamar.

Tamar was given to Er- Judah’s first son, who got offed by god for some undisclosed wickedness.so Judah tells his other son- Onan- to fulfill his obligation as Er’a brother and get Tamar preggers.

The idea here is that Onan would give Er and heir, who would inherit most of Judah’s shit. Onan, being not entirely stupid understood that if Tamar never gave birth to a son, then the greedy fuck would instead inherit Judah’s shit.

So Onan gamed the system with his pull out game, and god was like, “you little shit!” And smoked the greedy fucker. It had nothing to do with Onan pulling out and everything to do with him being greedy. If he tried the rhythm method, he’d have still gotten smoked...

And all this because the best social safety nets for single mother’s they could come up with was “hey. If the brother gets her pregnant, a son could be an heir and then she could take care of everything until the son is old enough!” Because women were property and couldn’t inherit anything.

I actually don’t think the bible has a compelling stance on this subject. The only relevant things the bible had to say that I can think of are that sex is restricted to marriage and the stuff about not spilling seed. So… the reason a Christian couldn’t have sex with roadkill is because they aren’t married to it or because it can’t get pregnant? That’s a pretty weak argument.

Leviticus 18:23: “You must not have sexual relations with any animal, defiling yourself with it; a woman must not present herself to an animal to mate with it; that is a perversion.”

To be clear, I'm an atheist and I don't fuck roadkill. But I do think this covers it from a biblical standpoint -- I don't think it makes any exception for dead animals.

Then again, this comes from some of the same passages that contain things modern Christians routinely ignore like "don't eat pork and shellfish" and "don't wear mixed fabrics". If they can ignore those rules, why not the animal-fucking rule as well?

Funnily, this drives at a niche thought experiment in vegan circles: "Is eating roadkill vegan?" That is, is veganism not fully a subset of a plant-based diet*? The Vegan Society's definition is roughly what most vegans would go by even if they've never heard it:

Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment.

To be absolutely clear, no vegan takes this thought experiment seriously. Vegans would prefer the animal be left alone to be consumed by e.g. other animals, and some hypothetical vegan delusional enough to pull that card would be ostracized immediately and plausibly worse than if they'd bought meat to eat for themselves.

* Notably, no, it isn't. Regardless of this particular scenario, there are edge case scenarios you can come up with that vegans would consider vegan but are definitionally not plant-based – because veganism stipulates "as far as is possible and practicable" while a PBD is a bright line.