At least Mofatt has admitted he's writing fanfiction and told fans he has no problem with slash fic, even went so far as to say some were really good.
Your last bit there...I honestly never thought of it that way, but you are right I think. Expanding on it: Women don't really have porn aimed at them. So some feel the need to create their own. For a lot of women, the reason porn in general doesn't do anything for us is because we need emotional connection in order to get what we want out of it; two random people fucking isn't the same as an epic romance with angst and UST and two fully realized characters. Slash happens a lot because the male characters are more fully realized; we know them better, they have better storylines and development. For shows like Sherlock, it's either that or...Molly? Mrs. Hudson? Heh.
If they are hot, bonus
I find it super odd that men are totally fine with the fact that a lot of men like lesbian porn, but are SHOCKED and appalled when they find out about slash. It's very weird.