HOT NUMBERS: You'll hear more than heavy breathing on public-access porn channels: Classic rock everything from Rod Stewart's ''Da Ya Think I'm Sexy?'' to Diana Ross' ''Ain't No Mountain High Enough'' is also being piped in, though no one is being paid any royalties. ''Rod is neither amused nor flattered,'' says the singer's manager, Arnold Stiefel. ''Our lawyers will move quickly.'' Not every artist is getting hot and bothered. Def Leppard guitarist Phil Collen thinks the use of ''I Wanna Touch You'' in sex ads ''might be a good omen. Strippers were using 'Pour Some Sugar on Me' before it was released,'' and it went to the top of the charts.