Slash in Star Wars

I just don't see the point. In both of the trilogies we have a trio of main characters, two men and a woman. In both, one of the men is in a canonical relationship with the woman. And yet we have Obi/Ani and Han/Luke slash. It's not just a question of 'This character is canonically straight' (and the rejoinder to that is 'Ever heard of bisexuals?'), but, 'this character is canonically in a relationship with someone else'. It's irrelevant whether Anakin or Han like men as well as women, because they've each chosen a particular woman. I'd be equally annoyed if someone put them in a relationship with a Mary Sue female.
And Obi-Wan/Anakin annoys me the most, I think. Both of them see their relationship as familial, and, frankly, I can't see Obi-Wan having sex with anyone, let alone his Padawan. Making it merely about sex (ugh) would flatten out the interesting relationship as it actually is: Anakin trying to treat Obi-Wan as his father-figure while Obi sees him as a brother; fernwithy brought this to my attention the other day. Their differing expectations are part of the problem with them. Anakin needs someone to fill the dad-shaped space, and guess who's all ready to step up? Yup, Palpatine.