Well, while plausible, it isn't very likely.
While on active duty I was on a court martial board for a guy that popped positive for cocaine on a random piss test.
His defense was similiar, he claimed his GF used it on her nether regions and he must of ingested it during the act of cunnilingus.
He had his lawyer, a real one, not a sea lawyer, but he should have gone with the sea lawyer who thought up this defense.... but I digress.
The Navy.mil had chemists and other speakers of technical jargon, talking things like gas spectrometer, cut-off limits, absorbation rates, etc.
In the end, the "coke-snatcher" as we lovingly called him was awarded a big chicken dinner (BCD) a few days of hard time and forfeiture of all pay and allowances.
The moral to this was two fold, don't ask for an enlisted court martial board because you think you will get off easier, and sometimes it is best to make sure the plate is clean
before licking it.
bob