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3.
The same as an anniversary, but for the first time you ever have sex or fo the first time you ever have sex with the person you are currently dating, married to, etc.
(if your first relationship is when you first had sex, then it would be a double entendre)
1. the 12th this month is my sexiversary for the first time I ever fucked someone, but the 10th of next month is my (girlfriend's/boyfriend's/spouse's) and my sexiversary for the first time we've ever had sex.
2. yesterday was my (girlfriend's/boyfriend's) and my sexiversary.
by obamapproved September 11, 2009
 
1.
The date on which two people first had sex.
He was invited out for a drink after work, but he knew that since it was their sexiversary and she would be expecting him to be on his game, he declined.
by Kate Charlton June 18, 2008
 
2.
The date on which two people first had sex.
He was invited out for a drink after work, but he knew that since it was their sexiversary and she would be expecting him to be on his game, he declined.
by Kate Charlton June 18, 2008