No. Fanfiction involving sexual content between characters is often not appropriate as it violates the boundaries of the original work and can be disrespectful to the source material and its fans. Moreover, it may also make many people uncomfortable.
Definitely not. Fanfiction should be a form of creative exploration that stays within the ethical and moral boundaries. Sexual fanfiction about 'sam and dean' is inappropriate because it objectifies the characters in a way that is not in line with the spirit of the original work. It can also have a negative impact on the overall fan community by making it a less inclusive and more uncomfortable place for fans who don't want to be exposed to such content.
It's not appropriate at all. 'Sam and Dean' are characters from a particular show or story, and creating such sexual fanfiction goes against the intended nature of the characters and the general family - friendly or action - oriented theme that usually surrounds them. It also shows a lack of respect for the creative vision of the original creators.