"Summer Vacation" by Ann Douglas (annd@pop.tiac.net). Michael has been staying for the summer months with his Aunt Karyn, a rubenesque widow related to him by marriage, who is both an attractive lady and a pillar of the local community. One day when he comes home unexpectedly, Michael discovers that his aunt isn't exactly a prude, since she has the delicatessen owner's salami buried deep inside her pussy.
As you probably know, Ann Douglas writes stories that have two major characteristics: (1) there is almost always a good plot that contains some fairly hot sex, and (2) the characters often engage in some sort of "moral immorality." That is, they often break a taboo of some sort, but the author develops a rationale other than simple-minded urge to violate the taboo.
In this case we have an example of an older woman having sexual relations with a 17-year-old boy. But instead of having the older woman say, "Hey! There's a teenager! I think I'll seduce him!", we get a much more sensible lead-in, which lets us respect both the boy and the woman on the morning after.
Oddly enough, even though I have given you the details in the preceding paragraph, I still have not told you much about the story. You can still read it and enjoy the interesting twists and turns of the plot.