I seem to want to write what I'd like to read, and I'd found so little fanfiction that had a mixture of het and slash couples, which seemed a lot more like "real life" to me.
So, I started writing it in Gundam Wing, which has such a large cast of characters, male and female, that you can mix em up anyway you like.

In some fandoms, that obviously doesn't work, such as where the focus is on two characters, mainly.
When the slash pairing is obvious, I go with the flow. I started out writing het though, because the fandom I first was interested in has obvious het pairings.
I'm not much of a smut writer anyway, even when I write sex, I tend to focus on dialog and characterization. LOL I can write two people talking about sex much better than them actually having it, I think.