To bag and tag someone means to literally put a bag over their face and then engage in sexual intercourse (Tag-it). Bag and Tag is used when the person has a smoking body but a really ugly face (kind of like it was beaten repeatedly with a tennis racket). Bagging and Tagging can also work when you are bored of screwing the same person and you place a bag over their face and pretend that its someone else.
Bob "Dude, I'm sooo tired"
Joe "That's not what your mom said last night"
Dude "Dude, my mom is old and ugly"
Joe "Bag it and tag it"