Oh friend, this is one of the Great Recurring Questions. Many explanations have developed over the years; you can find one of the first in the landmark essay “Normal Female Interest in Men Bonking,” and I collated a long and wonderful Tumblr discussion about it, “On Enjoying Queer Erotica,” a while back. Nowadays many people say that women write m/m sex because of internalized misogyny. I don’t buy that argument completely; I think there are other things going on. But there’s another long Tumblr thread here which articulates that point of view, and the damage internalized misogyny can do in fandom, and includes some very useful remarks from a couple of self-identified lesbians.
I’ve put together a “Very Select Bibliography on Slash,” which gives more scholarly background on the issue. I’d also recommend saathi1013’s marvelous meta page, which talks about similar issues in fandom more generally. Thanks for the question, and I hope this helps!
After many years debating, I’ve concluded that it’s very attractive to read about other people’s arousal because it’s removed from mine. Kind of like how I’m attracted to fantasy novels because in a way, it’s sometimes easier to relate to something that’s further from real life.
You pay less attention to what’s happening and how you’d feel if that were happening to you, and more to how it feels to these fictional characters. It lets me give up the pressure of being responsible for my sexuality in my fantasy life. I can put myself in someone else’s body and feel their pleasure.
Also, from a practical standpoint it’s easier to describe what dicks do when they’re happy.
—a lesbian slash writer