I've thought about this a lot. I've written both gay and lesbian romance and although my lesbian romance gets far better reviews and nominated for far more awards I don't sell as many copies. Period. People aren't as interested in reading it. Even when I do give-always people are far more likely to want a copy of my gay romance novella than my lesbian romance novella.
A lot of people say romance readers are straight women so of course they are naturally going to be more interested in books with hot men. I think that is the easiest, least complicated, least threatening way of looking at the issue though.
I also don't really buy it, never have.
What i really think is that women have a hard time thinking about women; about female bodies, female sexuality and female characters in general.
Our society tells us female sexuality is dirty, wrong and shameful. Always. It tells us the female body is only ever not shameful or acceptably sexual when a man is looking at it. When a woman looks at herself, she is conditioned to only see the flaws, only see the ways she isn't attractive, only feel ashamed.
In her article about why female fans hate female characters more than male characters "For All the Women I Have Loved Who Were Dragged Through the Mud" Aiffe writes:
"Women project the standards society has put on them. If they’re told they’re annoying for talking about their feelings, they’ll think other women are annoying when they talk about their feelings. It’s a continuous cycle of policing. I think there is a certain degree of truth to this. Women absorb the social rules of what women are and aren’t allowed to be (spoiler: it’s all contradictory and we’re not allowed to anything) and judge other women by those rules. She’s annoying when she speaks, her voice is too shrill, she’s too meek and quiet and passive, she’s too rude and direct."
I think for a lot of women reading lesbian romance makes them have to confront their own anxieties and insecurities about their bodies and their sexualities as well as other women's bodies and sexualities.
If both of the main characters are women and you find yourself having a negative reaction to them or to their gender and sexuality, you might find yourself questioning why? Is it something about you? Something about them? Something about women in general? It might make you stumble upon a whole new level of internalized misogyny you didn't realize you had.
That can be deeply frightening and off-putting. Not something a lot of people want when reading a romance novel.
It is so much easier to think about male bodies and male sexualities which are constructed as natural, normal and overwhelmingly positive. Sexualized cisgender male bodies are not associated with the same kind of body policing or shaming (this isn't actually completely true for all men but generally the kinds of men who are policed and shamed don't get romance novels written about them even in m/m romance) that sexualized female bodies are.
I think this is also where some of the backlash against 'strong female characters' comes from. Anytime this subject get's brought up in the writing community someone always pops up (almost always a woman) to tell me "not all women are strong" and "we need to write stories about non-strong women too." It has happened so frequently at this point that I think it's moved past the point of critiquing the way Hollywood has constructed "the strong female character" (which I think genuinely does need to be critiqued). The conversation is hardly ever framed as "the way the Strong Female Character is constructed in say Hollywood action films or the fantasy genre is problemtic" instead it it almost always portrayed as "strong women as characters are problemtic." This distinction has caused me to wonder if a lot of women get triggered by any kind of talk of strong female characters because they themselves don't feel strong or don't consider themselves strong and its anxiety inducing for them to have what makes a woman 'strong' talked about at all.
In the same way I think for a lot of women it's triggering to see women portrayed as confident and sexual without having men involved. It brings up, all of their own insecurities about their bodies and their sexualities. It highlights all of the ways they've been told that they are bodies aren't good enough and their sexual desires are wrong without the 'safe space' of a male body or male sexuality to retreat to.
But there is also I think another layer to the question of lesbian romance vs. gay romance and why one is so much more popular than the other.
I also think it also has to do with the way (white, cisgender, able bodied, middle class) gay male identity is portrayed in the media and in Western society.
Because m/m romance does better than lesbian romance, and bisexual romance, and trans* romance. It just does better period. Also in the m/m romance community the men being represented are almost always in white, able bodied, middle class men who conform to a normative standard of physical male beauty. There aren't a whole lot of chubby guys in m/m romance, disabled guys, working class guys (unless he is falling for a millionaire in which case the class problem will be 'fixed' by the end of the book when he marries into privilege.) Gay guys of color are often not represented and forget about trans guys. That can't be a coincidence.
In his article "The Myth of the Fag Hag and the Dirty Secrets of the Gay Male Subculture" Rohin Guha writes:
"Popular culture was teaching newly-out gay men that they could be welcomed into the heteronormative fold so long as they shoehorned themselves into these pre-approved [media constructed] molds of gay male identity. "
Basically he argued that the media and society has created a gay identity that is acceptable and non-threatening to heteronormative culture. These are the kinds of gay characters even otherwise homophobic Americans enjoy seeing as bit roles in tv shows like Will & Grace.
Lisa Duggan has pioneered the concept of homonormativity which is:
"a politics that does not contest dominant heteronormative assumptions and institutions, but upholds and sustains them, while promising the possibility of a demobilized gay constituency and a privatized, depoliticized gay culture anchored in domesticity and consumption"  (emphasis mine)
Of course this heteronormatively acceptable homosexuality or homonormativity is really only available to white, cisgender gay men who conform to the acceptable stereotypes of a gay man. But these 'acceptable types of gay masculinity' are exactly what the overwhelming majority of m/m romance novels promote. They are comfortable and non-threatening images of gayness easily consumed by an audience that might even be, in many ways, homophobic. People can feel good about being "supportive allies" to the GLBTQ community through consuming these images of very normative white, able bodied young men. Whiles these images also mean that they never have to question any of the deeper homophobic, biphobic or transphobic views they might still hold.
Lesbian or other queer women on the other hand, along with any and all trans* people and QPOC, are still threatening to heteronormativity. These unacceptable forms of queerness are just not as easy or comfortable for a large part of a wider Western audience to consume.
I think that, coupled with a lot of the internalized anxiety and shame women feel about female bodies and female desires, makes lesbian romance or romances that depict queer women significantly less popular.
Thank you to everyone who read over this first and encouraged me to post it.
1. Luibhéid, Eithne. "Queer/Migration: An Unruly Body of Scholarship." GLQ: A Journal of Lesbian and Gay Studies, Volume 14, Number 2-3, 2008, pp. 169-190 (Article) Published by Duke University Press
2. this needs to be an article in and of itself but I haven't written it yet, because it would involve a long discussion of the hierarchy within gay politics, assimilation, and representation.