I have read into this subject extensively, and I have discovered just what exactly is the absolute sexiest turn-on in regards to this genre of erotic literature. I'm getting ahead of myself however.
Passionate Son Voyeur is the genre of erotic literature which describes a scenario where the son partakes in seeing his mother get fucked by another person.
This other person, as I have discovered for many reasons, is best played by a son's friend who is passionately attracted to the mother. The reasons include a heightened sense of taboo when the event of sex finally arises, a natural and easy segway into beginning the relationship, and a realistic reason for the man having a big dick and a massive libido.
The mother can be single, or married. In the case of the mother being married, it should be a bit harder for the son's friend to bone her, but in either case it should require at least 3 or 4 meetings or dates before anything sexual happens. This includes hand jobs, making out, blowjobs or fucking.
The ideal erotic story in this genre needs to both convince the reader of the legitimacy of the story (for example, don't have the son super turned on by his mother, have the son be spiteful that this other individual is boning his mom (NOT JEALOUS!)) and also surprise the reader with a sense of taboo which arises in the final scene where the son's friend finally fucks her.
The mother should be the one getting seduced! I cannot stress this enough! It is entirely in human nature for the guy to want to fuck the living hell out of the chick, and especially in the case of the mom and son's friend being the two specific individuals in this case, the woman's feelings for the son's friend should be repressed almost entirely! The mom should be flirty, and allow the son's friend to get a bit touchy (again, after 3-4 meetings/dates), but the sex should always be surprising for the mom. She should always be a little ashamed of her self right before they have sex (ideally when they are both naked, and/or she is in the process of giving the guy a blowjob), but this shame should go away once the son's friend finally penetrates her.
I have also come to the conclusion that in reality, the son would be quick to cock-block the son's friend and his mom, so there must be a reasonable explanation for why he cannot interfere. The most typically successful one is a wager or bet between the son and his friend.
Now we're getting to the important bit: Once a sense of reality is established, and that the son's friend is hitting on the mother, who is sexually repressed, but feels she cannot do anything sexual with the son's friend, the taboo of the two individuals actually developing a passionate relationship should wow the reader.
A few other important details: Don't make the son a wimp. Don't have the mother and son's friend humiliate him in a sexual manner, or at all really. Remember, this is his MOTHER, and his FRIEND. If the son is humiliated at all, it will be because he sees his MOTHER slowly but eventually give into his friend's advances, upon which the ultimate sex ensues. Don't have the son have an incredibly small penis. Make the son's dick a realistic size (5-6 inches), while the son's friend can be above average, but not unrealistic (7-9).
When the ultimate sex eventually happens, the two individuals love making should be ideally passionate but lustful. There's nothing wrong with the son's friend saying that she's "so fucking hot", but calling her a bitch or whore just destroys the sense of anything pure between them.
The structure of the story would thereby be:
3-4 meetings of innocent flirting, playful touching, little things that the son (remember this is his perspective) will keep questioning the legitimacy of.
1-2 meetings of somewhat obvious sexual tensions. In this case, the mom should continue to act oblivious of the son's friend's advances, but allow more feeling, more touching. As an added bonus, a poolside scene where the son's friend gets to slowly advance his hands (while applying sun lotion) to rub the mom's tits before being told to settle down is great! You shouldn't have any making out or actual sexual actions here.
1 ultimate sex scene where all the lustful feelings and passion just boil out. Remember, the mom is a human being capable of understanding her limits. She should be okay with making out passionately, start to get a little nervous / embarrassed / ashamed once they're both naked, but absolutely go wild once the penetration actually starts. Realism is key!
The only deviation from reality in the story should be first and most obviously, the scenario itself, and second, as in all forms of pornography, condoms just act as killing the mood, so they are unnecessary.
To heighten the passion between the son's friend and mother, which by the time of the ultimate sex scene should be full blown love, the son's friend should cream pie her at least once. As an added bonus to represent the tantamount love the two individuals now share, this can end up impregnating her if the author so chooses.
Finally, it must be suggested that their love is legitimate towards the end, in that if the mom was married, she divorces her neglectful husband (who should always be rich such that the mom cannot be allowed to fall into a harsh economic time) and continues either dating, or marries the son's friend.
Now for the theory of why the above story structure is awesome:
The true eroticism we feel for a story is in its fantasy. We derive the hottest and sexiest feelings from our perfect little fantasies. It comes to no surprise however, that as fantasies, if the fantasy sex is realized time and time again, we become unconvinced of the possibility or legitimacy of the fantasy, and this is a major mood killer.
The structure of the story described above is such that there is a realistic beginning, which progressively and realistically finally realizes the fantasy. Because of the massive build up and foreplay, the eroticism in the single sex scene is entirely enough to satisfy the reader.
In essence, the fantasy is shown to be derived from the reality, which is in and of itself one of the sexiest things that can be done with literature.
However, this sort of fantasy writing cannot be achieved in a single page on literotica. The nature of the necessary build up requires it to be at least 2 1/2 to 4 pages long. However, it creates an epic to be used for personal pleasure until the reader memorizes each and every sentence. This is the nature of only the very best erotic stories. My fetish should be obvious by now: I like son voyeur with a particular focus on the friend. He is the important character in my fetish, where the POV is from the son.
Only a select few of the stories here I have read that include this fetish have been enough to please me time and time again, where others get old. These include:
1. The Bet by TheStinger - http://www.literotica.com/s/the-bet-53
This masterpiece right here is perhaps the greatest erotic story I have ever read which puts my fetish to work in the best structure I have ever seen. The only reason I'm even writing this instead of masturbating to The Bet is because I have practically memorized the sequence of paragraphs in this story.
2. Dad's lost Bet by TheHumpman - http://www.literotica.com/s/dads-lost-bet
This one is great. It deviates from the passion/love pattern with instead having the sex be acts of spite for the mother's husband and son. In this regard, the story retains an intense feeling of realism (or at least a realistic development which resolves to a fantasy end). It also succeeds as the others in this list do by having the sex occur exactly ONCE. Additionally, this spite for others rather than love scheme works perfectly for the somewhat shorter format.
3. Only Mom and I Know by thestoryspinner - http://www.literotica.com/s/only-mom-and-i-know-ch-01
This story works excellently because it shows through realistic terms how the relationship between the mother and the son's friend gets to a certain sexual level. It then perfectly displays the mother's reluctance to be unfaithful and have actual sex with her son's friend, followed by giving into the son's friend, and in fact, eventually giving the reader a sense that the woman was deeply sexually repressed, and shares an extremely lustful fuck with the son's friend all at once. It feeds the reader the idea that she is in love with the son's friend's dick without seeming unrealistic.
4. Home Run by Sa2222 - http://www.literotica.com/s/home-run-4
This is perhaps what inspired The Bet by TheStinger, and it is an excellent piece of work in its own regard. It places fourth because I personally hate to see assholes get the girl, it kills the mood slightly. Furthermore, the reason it is worse than The Bet by TheStinger, is because it fails to establish passionate love making. It gets the lust down perfectly though. Great story.
If you are also interested in this fetish, and see and understand the awesomeness that is this peculiar erotic story structure, my only hope is that this thread inspired you to write your own erotic epic, to last a hundred masturbations and never get old. Cheers to you mates.