Green’s Dictionary of Slang

(orig. US preppie/campus) sexual intercourse; thus parallel park v.

1981		Z-Link 30 Oct.–1 Nov. 🌐 Parallel parking inside and out.
1986		Eble Campus Sl. Mar. 7: parallel park – have sex.
1996		K. Lette Mad Cows 119: You’re parallel parking [...] A team cream. You’re having a bloody affair!
2002		Vocabula Rev. Jul. 4 No. 7 🌐 There are many jocular noun constructions that also provide the same meaning, and range in acceptability, although none of them has the versatility of their one-word equivalents: ‘Ugandan affairs,’ ‘country matters,’ ‘parallel parking,’ [...] as the allusion retreats from sex and approaches humor, it becomes more acceptable.