The worst thing about slash is that it is usually so improbable.
The thing is, most fanfic is improbable. Not because it's fanfic, but because many writers simply don't care to make their stories probable.
The majority of slash-fiction is utter crap, but then, the majority of all fanfiction is utter crap. While same-sex pairing often have little onscreen support, the same can be said for many hetero-pairings that prevail in fanfics. Many slash stories are about sex/romance and so are many het-stories. Just look at Picard/Crusher and Riker/Troi fanfics, the majority of those are about sex and romance.
There's just as much onscreen justification for Worf/Riker pairing as there is for Troi/LaForge. Hell, you could even make a strong argument that Picard/Crusher has less of a chance of happening than Picard/Q.
It also depends on what you want out of your fiction. Some enjoy whimsical drabbles, others prefer long action/adventure stories, or enjoy a story more when its focus is on romance. Neither is any more or less fanfic than the other, since fanfic, by nature, is anything written by a fan (or casual viewer for that matter) involving characters, items or universes created by someone else. Since there are so many different fanfiction-writers, there are many different stories. And as someone mentioned upthread, different interpretations of characters. As for slash, even if a character has only slept with members of the opposite sex on screen, that doesn't lead to being 100% hetero. There are many people today who don't realize an attracting to someone of the same sex until they're in their 40s or 50s, after having a lifetime of straight relationships.
Does that mean you can toss any two characters together without explanation? Yes. It's fanfic after all, you can do whatever the hell you want with the characters.

However, if you want to create story that realisticly portrays a relationship, this isn't the way to go, whether it's Picard/Crusher or Worf/Riker
In my opinion, you can only devide (Trek) fanfiction, in canon-categories. Ie:
*Could be canon: nothing on screen contradicts my story. For instance: Picard and Crusher have sex after, say, Devil's Due. It's never said onscreen that they
didn't have sex.
*Extra-canon: this is what you didn't see onscreen. Extra-canon tries to continue a scene after it's been cut off on tv. Take for example the scene in Second Chances, where Troi and Thomas Riker are rolling on the floor in the gym. We see them kiss and then the screen goes black. They could have sex, or not, we don't know and the show never tells us, so it's open for interpretation.
*In the face of canon: usually categorized under Alternate Universe, because these stories do clash with established canon. Any story for instance in which Picard and Crusher get married before Nemesis
*Canon for all we know: say someone writers a story about the crew set after Nemesis and adheres to all canon-facts established during the series and movies. You end up with a story that could happen and is canonically correct.
Anyway, those are just my two cents. Fanfic is wonderful because everyone can write what they want and not write what they want and everyone can read what they want, or not read what they want. But, a romance centered fanfic isn't any better or worse than an episodic story just because romance wasn't heavily featured on screen. Both are fanfics, they just attract different readers
