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A hookup or other episode of sex (such as the start of a new involvement) between people who dated or hooked up way back when, lost contact with each other for years, and became reacquainted.

See: http://thephoenix.com/Boston/Life/75527-Are-you-a-retrosexual/
She's not dating anyone right now, but she's had retrosex with two guys she hasn't seen since high school who she found on Facebook recently.
by my name is Cos January 25, 2009