Citations:gay it

(slang, now rare) to have sex

 * 1988, Thea Astley, Two by Astley, page 233:
 * Under the dousing of first-night perfume, too much of it, sneaking across the chemical trails of dry-cleaned dinner-jackets. The enemy is scattered. Five rows away I glimpse Georgie Shumway gaying it with a rotund fellow on her left.
 * 2010, Maureen Jennings, Except the Dying, page 195:
 * "I doubt she walked all the way from home without her boots." "I suppose she could have been gaying it in one of these cottages,” said the constable. “It's possible, but she's a long way from her own territory. I'm more inclined to think she came in a carriage. Maybe somebody wanted a winter poke."

References (for the meaning)

 * Eric Patridge, A Dictionary of Slang and Unconventional English (2006), page 450
 * John Farmer, Slang and Its Analogues Past and Present (1893), page 126