So, here’s the deal. Slash is a form of fan-fiction written that for the most part covers (male) homosexual relationships between two characters, from the same fantasy world, or even crossing boundaries (pun). Yaoi is anime/manga focused on homoeroticism.
So far, so good?
These are porn, and the characters written within them often fail to portray people of the gender they seemingly portray, not unlike lesbian pornorgraphy (pornography with two (or more!) women, mostly aimed at men, as distinct from pornography aimed at the lesbian community).
Now, I’m going to leave aside the discussion of whether Porn is or isn’t bad, but if we’re honest, fiction written in order to get people sexually aroused and which people use in order to get sexually aroused? I don’t really see any term to use to describe it aside from pornography.
My other claim which is a bit more controversial (that is less to say, it’s more of an opinion), is that there’s a “gender-swap” going on. The men in yaoi/slash, while they are males (biological distinction), are not men (social distinction). What they are, is women, or to put it in a crass but catchy manner – they are chicks with dicks.