Justin Bieber & Selena Gomez Duet Kissy-Face Karaoke

7/11/2011 6:24 PM PDT
This video should be rated "R" for Rats!!! ... which is what 13-year-old girls will say when they watch Justin Bieber kiss Selena Gomez after a romantic karaoke duet Sunday night in L.A.

Justin and Selena hit up Lucky Strike, Downtown -- where they performed their own rendition of the  Kid Rock/Sheryl Crow duet, "Picture."

But their curfew must have been pretty strict -- because J&S didn't linger ... hitting the road before midnight.

Oh, to be young ... and rich.  Screw young.  Rich.