Gary from 'Teen Mom' -- 'Amber Is Sluttish'

1/2/2011 12:00 PM PST BY TMZ STAFF
"Teen Mom" baby daddy Gary Shirley has a lot to learn about relationships -- like rule #1 ... don't call your baby mama a slut while on the phone with her cousin.

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In an audio recording from December obtained TMZ, Gary is heard on the phone with Krystal,  Amber Portwood's cousin. Gary explains to Krystal he kind of has a crush on her ... pointing out, "You've got things that Amber don't ... like a job."

Then Gary drops a real gem and tells Krystal, "Amber is sluttish. You've been with six dudes, Amber's been with f**king a million."

The topper ... dude actually tries to convince Krystal to come to his place by saying, "If you ever want to piss off Amber, let me know."

Mission accomplished.