Katharine McPhee Cheating Photos -- I Didn't Cheat On My Husband!!

Katharine McPhee insists she's no cheating skank -- claiming she was SEPARATED from her husband last year when she publicly made out with another man.

TMZ has obtained McPhee's divorce docs -- she lists her date of separation from husband Nicholas Cokas as March 15, 2013 ... more than 7 months before she openly locked lips with married "Smash" director Michael Morris.

As we reported, McPhee filed for divorce from Cokas last week. She cites irreconcilable differences in the docs.

Also interesting ... according to the docs, McPhee wants to keep all the money she made post-separation ... which might explain why she picked such an early separation date.

McPhee and Cokas briefly attempted a reconciliation in March ... but that just didn't work out.