Mayfly, from the 1/28/13 New Yorker
in the I will read and take brief notes on 300 short stories series
James and Molly, Jenny and Sam. Two couples, no happiness. On the drive to Jenny and Sam's mountain mansion somewhere away from Denver, James and Molly collide with a swarm of butterflies. The story centers on James and his misery, and how he and Jenny wallow together in misery of their mates when those mates drive down to Denver for the night.