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1.
When a group of girls wear different colored lipstick and suck a guys dick, leaving different colored rings on the dick. Creating a rainbow effect :)
Me and 4 chicks had rainbow sex last night! RED YELLOW GREEN AND BLUE!
by Shwanky240 July 13, 2009
 
2.
having sex with a girl on her rag
Hey I'm up for some rainbow sex. I don't mind.
by Sun May 10, 2006
 
3.
When people from different races have sex, calling it rainbow sex, because of all the different skin colours there are.
A dark person, a fair person, a caucasion person and a mixed person have an orgy, it is called rainbow sex
by superhungarishman April 06, 2010