I think the bottom line is simply that they're zombies. Zombies are fictional and you can do whatever you want with them. Stressing/thinking about how zombies should work makes less than the idea of them getting horny and so fourth. In my opinion it's best to just kick back and enjoy it. The art is cute & hot plus they're an adorable ship.
I'm surprised we're seeing Youmu playable again, though, I was expecting to see a newer game represented, like Rin from Subterranean Animism or Sagume from Legend of Lunatic Kingdom.
Not that I'm complaining, given that Youmu, Reisen, and Sakuya are all very well-liked characters, so more focus now means more doujins about them in the future.
Yuu’s reaction is just so relatable. As a marine life adorer myself, is very refreshing when someone randomly mention an interesting fact. Also, give Sayaka credit, people: intelligence is very attractive.