I remember reading a short story years ago about a microscopic race that was in decline, with a serious need for new genetic material to save them. They made a last ditch effort to contruct a minature woman "probe" to possibly go get some.
This minature woman appears in a guys apartment, and although she is only a few inches tall, they somehow manage to have sex. His ejaculation is transported back to the microscopic race as new genetic material to save them.
I'm pretty sure it was in an anthology paperback of various authors, and I would have read it sometime around 1998, so it would have been written/published prior to that. American publication, written in English.