Dollylux's Invisible Boy series is hot as burning (a slow, pressurized build, too, which is a plus) - Sam realizing his desire for Dean, watching him with a girl. (Dean may have left that door ajar by accident - or not)
http://archiveofourown.org/works/1017905

You don't care for threesomes - neither do I, they mostly squick me out - but nyxocity's Like Staring into the Sun is an exception. Technically a threesome, it is not about the girl(s). It's about Sam and Dean's journey toward each other. Strong element of voyeurism, even when both are involved.
http://archiveofourown.org/works/367041/chapters/596608

Take a look at leonidaslion's The Bright Lights of Disturbia. While Sam is at Stanford, Dean makes a porn film - which Sam accidentally watches. Dark fic, digs deeply into Dean's psyche, but it has a brighter ending.
http://archiveofourown.org/works/181007/chapters/266202

And for a fic that's almost 100% voyeuristic - in a general sense, although there's a hot sex scene or two in there - I love Frances Housman's poignant Dean Winchester, Lord. Amen. Lost in the River of Time, Sam resurfaces at various points in Dean's life to observe the moments and events he never knew. Really heartbreaking in places.
http://archiveofourown.org/works/1134627/chapters/2293636

I checked out the Voyeurism tag at AO3 - you're right, it's a mess! Lots of fic, with hundreds of pairings and partner-swappings - ewww.

Edited 2016-08-26 12:35 am (UTC)

Thank you so much!! Though I was looking for J2 fics but I'll surely check out your recs...especially the last two. Sounds amazing!!
And yeah....the tags in most of those voyeurism fics in ao3 are real scary...😳😉☺️

I don't have a strong heart to go through them yet. Lol

Hugs

Sorry. *headdesk* Completely missed the J2. I'll look again.

Edited 2016-08-26 09:28 pm (UTC)

There is Glittering Prizes and Endless Compromises by rockstarpeach on AO3.

Summary: Jared's in love with Jensen. The only problem is, Jensen's a hooker and Jared can't really afford him. It's a good thing he's so darn cute, because despite Jared's meagre finances, they strike up a business relationship that was only ever an excuse to become more than that. They're head over heels for each other, so it should be their happily ever after, right? Turns out it's harder than Jared expected, having a hooker for a boyfriend. Jensen on the other hand, saw this coming.

There is a scene where Jensen and his customer, JDM, let Jared watch.

Thank you so much!! Just finished reading it....awesome story.

Hugs

Glad to have helped!