Responses
Mpreg. I got him pregnant
Ok but joking aside, given the curse and how bad his communication skills are, its more likely he had a child with someone who experienced the tragic events in infinity castle. Trauma bonding and allat. And because of the
SPOILER
Lack of named female survivors, he probably got married to a background character. Or by descendant they could mean giyuu's relative's. Or kanao managed to make a cure for the curse. Or she unlocked the secrets of the universe and actually made mpreg possible. Idk. Author made it open enough for us to go wild with it

The parallelism here where Giyuu motivated Yae to live, hunts him now (karma). So I think there is a high chance Giyuu married Yae, coz the author opened the arc here where Giyuu is very depressed knowing of his time limit, but Tanjiro in the extra stated he looks more motivated now in the current letters. So Karma happened, might have been a moment where Yae is the one to motivate him. Also, I think he got depressed after his Hot Springs vacation with Uzui (who flirts with his wives). Its not hard to imagine since Kanao and Yushiro are both working for a cure.