Right. It was not legal at the time of filing, but there were indications that it could become legal through judicial action [1]. Prop 8 was trying to stop it from becoming legal.
[1] Prop 22 had been struck down by district courts, and then appellate courts overruled the district courts. The state Supreme Court agreed in December 2006 to consider the issue. Prop 8 was filed October 5, 2007. The Supreme Court did not rule until May 2008.
Proposition 8 was an initiave constitutional amendment filed in response to court cases striking down Proposition 22, an initiative statute with the same language as Prop. 8. Its true that there will still appeals pending in those cases when it was filed (though they were finalized before they Prop 8 qualified for the ballot.)