Since a siren's true form is a bird like creature then, according to fridge logic, sirens reproduce via eggs. This is also shown in ancient Greek mythologies where sirens create nests in the islands they live in. They kill unfortunate sailors to feed their young, like a bird.

In conclusion: SAN OTONASHI CAME FROM AN EGG!!! AND SHE WILL LAY A EGG IF SHE MATES!!! SHE WILL THEN SIT ON THE EGG TILL IT HATCHES!!!

Mother of god...