Short story: aliens that merge into one

[iodama]

I think I read this short story before 1989 in Asimov's SF Magazine (which started publication in 1971 and I was reading back through old ones in a collection, so it could be any time in the 70s or 80s). It concerned a young adult male protagonist (on Earth, who thinks he's human) who meets an "uncle" who looks a lot like him, from whom he learns that he is not human, but an alien passing among humans, who goes through a maturational process whereby two genetically related individuals fuse into one, and his "uncle" who is looking younger by the day and he are fated to merge into one person.

A key scene is the "uncle" taking the protag to see two other members of their species going through this process of fusion, and as I recall it there's a lot of screaming and moaning and maybe gory mess, and the protag says anxiously, "It looks like it hurts" and the uncle replies, "It always hurts being born."

Anybody recognize this story?