"Berries" by Uther Pendragon (anon584c@nyx.net).
http://www.dejanews.com/getdoc.xp?AN=344229979

Aurora and Lawrence are teenagers in love back in pioneer days in America. On this particular day they have been observing a bull mating with the local cows, and this has been giving Aurora and Lawrence ideas and urges of their own. Their passion is not alleviated when they suddenly find themselves watching a man and a woman from a wagon train frolicking in the woods. After watching, they move to a more private area and make love to each other.

But all of the action in the previous paragraph is actually part of a story that Larry is telling Dawn in modern times, while he fondles and caresses her. It's the ole sex story within a sex story technique, and the author brings it off very nicely.