Sonic Youth is post-no wave,
they are informed by No Wave (i.e. Glenn Branca's influence); they came too late for No Wave as it was a very short lived movement in the late 70s and very early 80s.
Yes, Sonic Youth didn't participate in the no-wave movement, but they were direct witnesses and had connections with a lot of those musicians. Ranaldo and Moore were already playing in small NYC bands (The Flux, The Coachmen, etc). Their first albums obviously reflect an influence, and Moore has always admired Lindsay's guitar approach, which was miles away from the neo-bossa stuff he's up to now.
Moreover Ronaldo played on "The ascension" : Glenn Branca's non Theoretical Girls No Wave album (which is a killer if you ask me), SY was produced by Warton tiers who was coming straight from the No Wave scene and when you listen to the very first SY record on SST or "Confusion is Sex" you can't help but think they're still into it at that point. Also of interest if you're looking in that No Wave direction is Public Image Limited's three first album, which aren't No Wave per se but still very in the aesthetic of this scene. By the way... Rought trade released a very in depth 2CD compilation which is called "Post Punk" which you might wanna check to discover stuffs from that era... if your'e also interest in Mutant Disco, Proto Rap, Post Funky Punk...etc there's always "Mutant Disco" and "New York Noise", both on Soul Jazz recordings, which feature both No Wave and other weird stuffs from this time. My two cents Olivier