@Draug: Trans women do get killed (and raped and disfigured) at a higher rate than the average woman (or average population) - Going into men's toilets puts them at risk of being outed.
I'm not backing down on my claim that this risk is worse than a little discomfort. (and note i'm not saying anyone who says they are trans, i am saying anyone who presents as female in their day-to-day lifes - taking to presenting as their identified gender daily is something which takes most trans people, but particularly trans-women, a long time because they don't want to go out unless they think they will pass as female - which has surprisingly little to do with the number of penises they have)
@smeck, the position of a philosophical skeptic is the standard position for an scientist. I don't see how taking the little science i know and talking about it is in any way countrary to that position.
- I will admit it is a hard area of science to study, but i also know that cis-gendered people don't often come out and change their gender identity as adults. While trans-gendered people often identify as the opposite gender before puberty (anecdotal)
@Draug, yes, the youth psychology is also a thing, as are XX female-identified tomboys, who go onto become women. But the fact that there is a parental and social influence doesn't change the fact that there is also a gender identity (though some people identify as gender fluid, or agendered) I identify as male, i happen to like women, and i also happen to have male sex organs. I'm about as lucky as i could be because being in the majority society readily accepts me as normal, that doesn't mean I should discriminate against those who are not so lucky.
There is still a horrible level of sexism going on, women have earned the priviledge of dressing like men (suits, pants etc) in public; but men are ridiculed for dressed like women in public. (with the exception of as a joke - which implies that men who want to be women are a joke) Trans-men are in a much better position, the can get away with presenting as male and if they don't pass they also don't suffer the same level of persecution.
And when it comes to toilets, I don't see the same scream to keep trans-men out of male toilets. Again, because it seems this is driven by a fear of the penis. It is penises that you want kept out of women's toilets/changing rooms/whatever. It is a fear of the terrible damage a penis will do to the women in these locations. Your daughters, sisters and mothers... This too is sexual discrimination. We don't hold our sons, brothers and fathers to the same standards. We require that the be manly enough to protect themselves.
Why is there so much fear of the penis? This is plain sexism, and sure you can 'fix' this double standard and hide it by passing a law which applies equally, but that doesn't mean you are an less sexist.
More to the point though, you are discriminating against trans-people when you happen to be in the lucky position of not being trans. A majority of males fighting to protect their females from the horrible sight.
And to be honest - "but it[THE DREADED PENIS] is still there for all the little girls who happen to be there with their moms changing to go swimming." - When i was a child and going to the local swimming pool with my parents, i often went into the womens changing room with my mom, or saw little girls in the men's changing rooms with their dads.
Guess what, it has never been a jot of a problem! You are making an issue out of this because you're scared, and when women bring this up as an issue I will be more likely to listen and consider it thougthfully. (of course in an ideal world my solution would be unisex bathroom/changing rooms, but alas we live far away from an ideal world...)