ill have to agree that clothing itself isnt so hot, but for me its more of the adventure that goes along with it. you can hide a lot of things in clothing, like those dom/sub plays where the dom puts a vibe somewhere in the subs erogenous zone and activates and changes the settings on it randomly throughout the day. they could be in a park and s/he could be doing their damndest not to make a mess of themselves right in front of you, but youd never know. or maybe i just got fucked in a movie theatre, and so when walking home, shes got a mixture of juices just catching in her panties, possibly for a gag later on ;) just things like that.
outdoors could have plenty of things going on for either end. full on naked is obvious, as its much harder to hide nudity, and is therefore hotter, but the discreetness of clothes means people could look right at you and just see a couple being romantic, when in reality the taker is getting reamed and both of them are trying not to moan out and ruin the secret.
