Not going to happen.
#1 - New.net has .xxx already
http://www.new.net/index_ext.tp?lang_id=english
which requires a plugin to view. If ICANN adds it to the existing TLDs, there would be legal conflict unless New.net has an agreement with them.
#2 - No webmaster with any sense would ever use it, since ISPs
can just filter *.xxx out from their connections, and block them completely.