Responses
I think the whole "one bride" thing is exclusive to mating with humans. The way I see it, mermaids in this story become mortal by sharing their immortality with mortal beings. Maybe after sharing once, mermaids no longer have viable immortal "energy" to share with another mortal. But that isn't enough to become mortal so the couple must entirely consume their shared immortal "energy" by conceiving a human child.
I’m sure the human and merperson both die after having child
If I'm understanding this right, if a human and merperson get together, the merperson gives the human its flesh to make the human immortal. After they conceive a child, the merperson is then able to grow old and die. The child is human too. This raises so many questions:
Does that mean that the human is now immortal (kind of sad) and outlives both the merperson and child, or is it that the human is only immortal until a child is conceived, making both the merperson and human mortal and able to grow old and die?
And does the only "one bride" only work with humans? If so, then where do merpeople come from? If 2 merpeople got together and their child is another merperson, doesn't that mean that the parents are still immortal (since the mortality thing only works with humans)? Or can the parents still conceive children with humans outside of their marriage and become mortal that way? But maybe not, because it seems like a given that the merman won't be taking another human lover if Kiyo dies (or maybe that's a personal choice).
Or maybe merpeople are able to asexually reproduce too.