Responses

Didn’t you read the manga? The cousin explained that a suitable mate can stable the condition of people like Ren. To stop him from “returning to the ancestor”. His family desperately made him taking the medicine everyday to wait for him to become of age. They chose a rabbit because they are good at that, but Kaede is the only one who is not married in his tribe.
Maybe his family is not fussy when it comes to finding a heir somewhere else. Adoption is an option.
There was no confirmation that they can have kids. The medicine used in Kataika simply adjusts Kaede’s body to Ren’s cos they are inter-species.