What we know so far about spirit reproduction:
1. It seems to require a male and female spirit, and the female spirit has the role to conceive the baby
2. It's a sacred event but not intimate (with some exception), unlike human marriage
3. Apparently age of consent for spirits are about 100 years old
4. It may not require a sexual intercourse to happen (maybe a ritual or some sort? Haven't been revealed at length so unsure)
5. Spirit are open to reproduce with more than one partner
6. Spirit offsprings seems to inherit one of their parents elemental attribute, and child care falls to the side of the parent that the child inherits their attribute from
7. Spirits seems to (traditionally) only view offspring of their own elemental attribute to be their child
8. Seems like there's not much limit on which element can reproduce with another element (ie any element can partner up with any other element, but seems like preference exist based on the spirit's personal taste)