Monumentally Stupid (http://strive2bhappy.livejournal.com/5224.html#cutid1) by strive2bhappy Season 7. Sam's hurt both hands, so Dean has to shave him. Having Dean that close to him, focused with that little concentrating frown, breath warm on his face, Dean's competent hands tilting his head and angling him exactly the way he wants him -- well, Sam's not doing too well right now pretending he's not attracted to his brother. And he's only wearing boxers, Dean's bound to notice. Dean's going to have him all figured out.
Dean is amused and turned on and there is a slow, sensuous first time with bottom!Sam.
Under Hill (http://intimations.org/fanfic/supernatural/Under%20Hill.html) by astolat (I've been dying to rec this). "Can’t we just kill a bunch of elves without having to talk about our feelings?" Dean said. Teensy bit of angst but is mostly funny and sweet and is a joy to read!
On the Cover of a Magazine (http://teashopmuses.livejournal.com/34029.html) by selecasharp Sam and Dean are called in to investigate the mysterious death of a model at a photography studio in Michigan. The only way for them to get in? Pose as models themselves – which is much easier said than done. Has fabulous art, a little bit of angst but is funny and romantic.

Season 7 first-time fics:

The Persian Version (http://archiveofourown.org/works/384019?view_adult=true) by de_nugis (and there's a timestamp here (http://de-nugis.livejournal.com/71049.html#cutid2))
Summary: Drunk!Dean appreciates drunk!Sam's smarts. Also his shoulders.

Planned Acts (http://laurificus.livejournal.com/183534.html) by laurificus
Summary: "I figure I've got at least forty-five minutes of optimism in me."

Extenuating Circumstances (http://laurificus.livejournal.com/181036.html#cutid1) by laurificus
Summary: While Sam was out, Dean was worrying about him, and possibly having epiphanies.
Note: This is non-explicit, and more pre-slash.

Season 8 first-time fic:

One Complication Less (http://laurificus.livejournal.com/190329.html#cutid1) by laurificus
Summary: "And in the third place, we're going to have sex."

I have 2 other first-time fics bookmarked for season 8, but they include a lot of arguing and you want more light-hearted stories, so I don't think they're what you want right now.

Don't have any recs, just wanted to say that this is an awesome request and exactly what I needed for a rainy weekend. Thanks for thinking of it!

Just want to say, I've enjoyed all these stories in the last 24 hours. So nice of you peeps to rec them!

Oh... I love first time stories and this is my favorite one... it´s not S07 or S08, it´s pre-Stanford one but... I´m sure you will love it :

1. absolutely awesome bewaretheides15 wrote "Crush" - http://bewaretheides15.livejournal.com/18463.html (http://bewaretheides15.livejournal.com/18463.html)
Summary - Dean looks at him different now, Sam just can't figure out what the difference is.

There is slow buid up and the love they have between them is just suffocating :) Beautiful first time story.

2. another perfect first time story is "Almost At Home" by balefully here : http://balefully.livejournal.com/598931.html (http://balefully.livejournal.com/598931.html)

3. "if we keep living this fast, no one will have time to die" by oxoniensis here : http://oxoniensis.livejournal.com/379020.html (http://oxoniensis.livejournal.com/379020.html)
Summary : First time, falling in love with a side of monster-hunting

4. Really short but really good one is "IT’S NOT GONNA BE JUST ONLY A KISS" by idlehands here : http://idlehands.ponderosa121.com/its-not-gonna-be-just-only-a-kiss/ (http://idlehands.ponderosa121.com/its-not-gonna-be-just-only-a-kiss/)

5. "Rest stop. Near Leland, Mississippi." by merryish you can find on AO3 here : http://archiveofourown.org/works/4208 (http://archiveofourown.org/works/4208)
Summary : A tiny story in which Sam and Dean kiss. For the first time.

6. Another one from merryish "three months | the kiss addiction remix" you can find here : http://archiveofourown.org/works/4203?view_adult=true (http://archiveofourown.org/works/4203?view_adult=true)

7. One beautiful first time called "A road trip down memory lane" by radiophile is here : http://lockthecolt.livejournal.com/33151.html (http://lockthecolt.livejournal.com/33151.html)
Summary : I'd take anything you give me, and you'd give me anything I'd take. | Sam reflects on their first time, back when things were easy between them. (pre-series up to S3)

8. And one absolutelly gorgeous first time "What Lies Between" by Sonny you can find here : http://samdean.archive.nu/viewstory.php?sid=3659

I hope you will enjoy them as much as I did ;)