Well, its come to this. I'm looking for a manga where there is spontaneous sex. It doesn't have to be in a bed, the seme could just pick up the uke, carry him off, and -boom-... there you go. It needs to be unplanned and unprepared for.

Examples
Viewfinder
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Junjou Romantica

Thanks! <3

What a good question!
I think you might like...

Black Sun Doreiou ...If you like rape. In front of everyone.
Mix Mix Chocolate Fun ensues when aphrodisiacs are introduced.
Takuhai Cupid Rape again, but more humorous. (We all know uke don’t actually mean “no” when they say it)
Aijin Incubus Random in the middle of class sex work for you? (Man, I have been namedropping this manga a lot recently)
Wanko to Nyanko Also spontaneous classroom sex. Very smutty.

Speaking of smut, you may enjoy Minami Haruka stuff in general.

[color=blue]THE SOUND OF THE OCEAN IS DEAD
[/color][color=gray]IT'S JUST THE ECHO OF THE BLOOD IN YOUR HEAD[/color]

Ano Kado wo Magatta Tokoro - everything starts because of a casual encounter, is really a lovley story.

[Bónd(z)](http://www.mangaupdates.com/series.html?id=3810) - I love the first story where two friends start to have casual sex without bigger commitments.

Kanga ne Spoon - hot scene in a fashion show bathroom. Really hot stuff.

Also most of Masara Minase stories have casual encounters.