I wrote a similar essay when I first started writing fan fiction, with a lot of the same points! It's so hard to cover everything that it could be, even when we try to figure out all the different mentalities that might bring people to slash.

One thing to take into account is that a lot of the people writing slash are not straight. There is a large group of women who identify as lesbian or bi, and quite a few in relationships with other women, who write m/m slash. There are also gay men writing and reading, and at least a few trans people who are out about it.

I think it's somewhat interesting that straight women like two cocks porn, but in some ways more unexpected that so many women in f/f relationships do.

It might be because the canonical literature from which we are working doesn't have such brilliant or attractive female characters. Or it could just be the ineffable, inexplicable human brain, enjoying erotic stimulation from things we don't like in real life.

I think the model of a lot of intersecting reasons works well.