Generate a pair. Then try to link the pair through actual, canonical sexual couplings, with as few degrees of separation as possible: Pairing A/D. A did B, who did C, who did D. Three degrees!
Example from the Buffyverse: Robin Wood, Spike. 4 degrees of sexual separation. Robin's done Faith, who's done Riley (while in Buffy's body, but still), who's done Buffy, who's done Spike.
Obviously this works best in more promiscuous 'verses. Pairings involving anyone who never got lucky in canon are automatically discarded as impossible.
We played a bit in my journal, here.