Although there are a few exceptions, the term has little if anything to do with biracial, citizen-of-the-world, kumbaya style of multiculturalism that the user would want their gay socialist friends to believe. It is most commonly used as a smokescreen to explain their spending part of their childhood overseas under the umbrella of the evil Republican imperialist war machine.
"OK, so like I was born in Okinawa and went to school in England, Germany and Guam."
"Wow, that's so cool. Was your mom in the Peace Corps or something?"
"Sort of... my dad was like with the government. My sister and I were like Third Culture Kids."