Since you believe that trans people are the gender with which they identify and wlw attracted to preop trans women aren't lesbians, unless you're saying that preop trans people on HRT are some sort of intersex (which brings up a significantly dicier conversation about attraction to intersex people), you're saying that everybody with a penis is male; since these standards ought to be symmeric, I'd assume the same is true on the other side, and you're saying that anybody with a vagina is female.
If you believe that sex is entirely determined by genitalia, please stop leading by saying that you don't. If you don't believe that, and aren't putting us in some "other" bin, I don't see how it scans to say that attraction to preop trans women precludes lesbianism, since it would be possible to be a female with a penis.
I suppose my argument that genitals don't define sex alone is that, similar to losing your uterus or ovaries to a hysterectomy, losing your breasts to a mastectomy, etc., you can lose your genitals (e.g. due to bad trauma, particularly bad cases of fgm), and I don't think this changes the sexuality of the people who find you attractive.
I'm not saying that pre-op trans people on HRT are some sort of intersex but I think it's disingenuous to say that they are female at that point, either. Hence you calling them fairly female. As a lesbian I am not attracted to fairly female people, I am attracted to females. If I was attracted to fairly female people that gives wiggle room in my sexual orientation and I would no longer be monosexual - attracted to only one sex. Being "fairly female" is not the same thing as being a cis woman or a post-op trans woman.
My sexuality is based on sex not just gender. Sex is made up of secondary sex characteristics and genitals. I am saying that you need both pieces to be finished in your transition from one sex to the other. Until then you remain as being, "fairly female" as you put it (and by extension, "fairly male" for trans men). And to say that genitals have no standing in my sexual orientation is insulting. There is a reason conversion therapy has a focus on genitals.
Losing your genitals is not the same thing as never having them, that is a false equivalency. Furthermore, a mutilated vagina or a vagina that has undergone trauma is still a vagina. A paralyzed arm doesn't suddenly stop being an arm and it is not comparable to somebody who was born without one.
Similar to the "lesbian = gender attraction" thing, I don't really know how to interact with the claim that people who are not attracted to people with penises are being pushed out of lesbian spaces. I understand your disdain for the action of kicking people out of lesbian spaces over having your genital preferences, I just think that it's objectionable to try to kick out people whose genital preferences don't align with yours.
Frankly, I think it makes no sense to push the idea that there's a slippery slope between these ideas. The people saying them are two different groups of people (and, to be honest, the "you need to reassess your genital preference" crowd is small and controversial within even most tucute spaces), and they don't occupy the same conceptual space.
I do not have a damn genital preference. I am a lesbian. Part of being a lesbian is being attracted to women with vaginas. If you are capable of being attracted to vagina's and penises. Then you aren't a lesbian. Because a penis is male genitalia regardless of whether it is on a trans woman. It doesn't suddenly become female genitals when a trans woman starts her transition. Much like how sex is made up of two puzzle pieces: secondary sex characteristics and genitals being a lesbian is made up of two puzzle pieces: being attracted to women (see: Not trans men) and vaginas.
The "you need to re-asses your genital preference" crowd is not as small or harmless as you think. You will be banned from the biggest WLW group on reddit for saying that you aren't interested in dick. You will be banned from the LGBT reddit group for saying you don't like dick. There are various lesbians who have recounted their experiences being shunned from in person lesbian/WLW and LGBT support groups because they do not like dick.
Not to be super aggro about it, but I'm getting kind of annoyed at this whole defining-sexuality-via-genitals thing in part because it hits the biphobic stereotype of the monster who can't stop munching dick & vagina to punt people to bisexuality the second they're ok with either type of genitals. I'm bisexual because I'm attracted to people regardless of their sex, and I do not want people in the bisexual community who can't relate to the experience of being attracted to somebody they view as a male.
That's because bisexual people are okay with either type of genitals?? I don't understand what you mean when you say that. By nature of being bisexual you are attracted to both sexes and all genders. That is not something that is extended to monosexual sexualities. That's not feeding into a, "dick & vagina munching monster" stereotype that's just what being bisexual means. Saying that you're capable of being attracted to both men and women, and by extension both male and female genitals, is not me saying you want to fuck every person who walks by. That would be like somebody saying a lesbian wants to fuck every woman they see, or a gay man wants to fuck every man they see - those are harmful stereotypes to paint us a predators. I am attracted to women - that doesn't mean that my attraction for them is uncontrollable.
Last thing, I promise, and it's the elephant in the room: my focus on this has been describing the phenomenon of being attracted to females and whether this includes preop trans women as targets of attraction. When it comes to the other side of the coin (including preop trans women in lesbian spaces), I'll just firmly say that it will not work to push them into social spaces built for heterosexual males.
Not to be too emotional or aggressive about it, but I really detest a system that forces my girlfriend to reassess her sexuality before seeking support over lesbophobia from family or coworkers, anti-lgbt violence happening in my city, or whatever else. Further, I won't support any system which forces me out of wlw spaces which are open to bisexuals on the basis of genitalia, since there's nowhere else for me to go and find people whose relationships share the same nature as the majority of mine have.
I am not pushing WLW who are attracted to pre-op trans women into heterosexual spaces. I am not pushing WLW out of WLW spaces - I am saying if you are bisexual than lesbian spaces are not your spaces. The same as how bisexual spaces are not lesbian spaces. Do both spaces have women who love other women? Yeah. But that doesn't make bisexual and lesbian women the same - the lack of attraction to men and male genitals is a big aspect of being a lesbian. That part of our sexuality is something we are constantly vilified for and that part of our sexuality is why we need a space solely for other women who only and exclusively love the same sex.
Lesbian spaces aren't the only WLW spaces in existence. Bisexual spaces exist, gender specific (so bisexual women) spaces exist, generalized WLW spaces which include both lesbians and bisexual women exist.