Let's begin with a selected reading from Mysterious Akavir:

"Akavir is the kingdom of the beasts. No Men or Mer live in Akavir, though Men once did. These Men, however, were eaten long ago by the vampiric Serpent Folk of Tsaesci."

You already know about the birds and the bees. Let me teach you about the cows and the snakes:

• Most mammals and some reptiles like pythons use a male-dominant XY sex determination system, where XX codes for default female development and XY overrides that to produce the male phenotype, usually via a major switch like our SRY gene. YY is degenerate and fatal in our species and most others.
• Most reptiles, including birds and most snakes, use a male-recessive ZW sex determination system, where ZZ codes for male phenotype and ZW codes for female. WW is also female, although this configuration can become degenerate (lethal or infertile) in some species.
• We can call the Y-dominant masculinization pathway "mammalian" and the ZZ-recessive masculinization pathway "reptilian" for simplicity, but that's not exactly accurate. They're broader than that, and some remarkable species use both. Complex combinations are biologically possible, and nature is known to switch it up from time to time.
• In most cases, the letters refer to system configuration (male-dominant or male-recessive), not to the actual shape of the chromosomes (which historically gave us X & Y).
• Systems vary in complexity depending on whether the masculinization genes (Y and/or Z) occupy the same chromosome (some fish) or live on separate chromosomes (some fish, amphibians, reptiles, insects). Lots of other systems exist, too. We'll only be discussing advanced species where the systems live separately, with an independent XY chromosome dynamic operating in a different location than the ZW chromosome dynamic.

Please note: This is not a discussion of DSD's (disorders of sexual development, typically resulting in infertile intersex phenotypes) or gender-dysphoric (trans) identities operating independently of chromosomal systems. Those are real-life complexities beyond the scope of this discussion, and should be discussed elsewhere please.

Parthenogenesis is the ability of some females to produce offspring without a male partner. Plants and some insects practice a form of parthenogenesis that produces true clones of the mother. However, parthenogenesis in vertebrates work differently, producing half-clones instead. Half-clones receive one set of recombined chromosomes from the mother, then duplicate that set to form their second set (instead of inheriting a distinct set). The resulting organism is maximally 100% inbred, homozygotic for everything, and therefore all recessive traits will be expressed. Many will therefore not survive development, or inherit health problems. Many reptiles are capable of parthenogenesis, including pythons (XY) and rat snakes (ZW). Since they use different systems for sex determination, the results are dramatically different: female pythons will always produce XX female half-clones, but rat snake females only produce ZZ male half-clones (WW is lethal in that species).

So consider A Tale of Two Serpents who wash up on a snakeless island together: a female python and female rat snake. The python could successfully populate the island with female half-clones, but these would never be able to reshuffle their genetic material without a male present. Each first gen daughter would be a half-clone, then all subsequent generations are effectively full clones of the half-clones that were healthy enough to reach reproductive maturity. The rat snake would produce male half-clones, and those healthy enough to reach reproductive maturity and pass inspection could then mate with the mother, producing both males and females. Normally, inbreeding is inadvisable, but in this case the homozygotic male is a suitable breeding partner, already selectively filtered to eliminate any recessive traits that would cause trouble during development or before reproductive maturity. This population could produce both males and females, restoring sexual mechanisms, and reshuffling material with each generation, building diversity and accelerating adaptation to their new environment. Takeaway: in species retaining parthenogenesis, the ZW system is biologically superior at colonization from a single founder.

Real humans are mammals using the XY sex determination system. Although parthenogenesis was lost in our lineage, normal mammalian human females are XX-WW, therefore any event of human parthenogenesis would produce only XX-WW female offspring if we had retained that ability. For any human parthenogenesis event to produce male offspring, that would require the mother to belong to a unique lineage of "reptilian" females possessing the XX-ZW genotype. The resulting male half-clone would not be an XY mammalian pathway male, but an XX-ZZ reptilian pathway male. A serpent-man. Like the Tsaesci. Such "reptilian" humans would have entirely human autosomes, but operate on a different sex-determination system with complex cross-compatibility. They would be humans, but... parahumans. Our chromosome 9 is homologous to the set of genes that govern the reptilian pathway and parthenogenesis, and is the chromosome most likely to be recruited if such a ZW system existed in any human population (due to a Daedric or divine serpent seed curse).

Now that we understand the biological foundation for one species to contain four complex sex forms (and again, some in nature do), let's spell out all possibilities explicitly: the basic four combinations are XX-WW mammalian female, XY-WW mammalian male, XX-ZW reptilian female, XX-ZZ reptilian male. If the regimes breed within their own category, then sexual reproduction works normally, producing 50/50 male/female offspring of the same regime. But crossing regimes can get strange: reptilian males crossing with mammalian females can only produce reptilian females (XX-ZZ × XX-WW = XX-ZW). And there are two additional distinct sex forms that can emerge only from crossing-breeding mammalian males with reptilian females, in species where these combinations are viable: XY-ZW male reptilian-carriers, who in turn (if fertile) can produce XY-ZZ supermales with both masculinization pathways simultaneously activated. Altogether, the composite XY-ZW sex determination system has six possible sex forms: two female (mammalian & reptilian) and four male (mammalian, reptilian, reptilian carrier, and reptilian supermale). Reptilian carriers are the only reptilian males that could produce mammalian offspring, only by crossing with mammalian females.

All that being said, there are 23 possible results from a crossing event between these 6 sex forms. 8/23 results are mammalian, while the other 15/23 results are reptilian. Meaning: the reptilian masculinization pathway could enjoy a demographic advantage in the rare species having both forms, depending on relative fertility. Emerging Z-reptilian strains have an inherently subversive tactical strategy of concealing their existence, invisibly infiltrating, and demographically overwhelming an unsuspecting host population. This provides a biological explanation for Tsaesci serpent-men eating the men of Akavir, not transmissible "vampirism" but heritable rivalry, an emergent consequence of competing reproductive strategies. (Sure, you can hand-wave any magic explanation you want, but this is an exercise in grounding lore to reality.) And it provides an explanation for 3 distinct forms of Tsaesci male: normal reptilian males, reptilian carrier males (mammalian-passing appearance), and reptilian supermales (hyper-masculinized XY-ZZ Snake Kings). It also provides phenomenal story potential for a Tsaesci "Secret Wars" conflict exploding across Tamriel.

Hypermasculinization via dual pathways could also explain the true nature of Argonian Behemoths, and Naga could be the "mammalian" version of Argonian genetic expression if they use a similar composite sex determination system.

TLDR: This discussion is about mammalian vs reptilian pathways of masculinization and how they could provide a real-world biological basis informing game lore such as the Tsaesci serpent-men racial/curse dynamic. Out of the two options, it makes the most "biological sense" to implement playable Tsaesci serpent-man heritage at character creation (via a new race), not from a bite (new curse). It could be a racial curse from Boethiah, like the Orc/Malacath and Dunmer/Azura curses. An extra playable male sex option differentiating normal Tsaesci reptilian males from Akaviri human-passing carriers would be biologically sensible and world-enriching.