The bathroom is made for people with the same parts so transsexual people should have to get bottom surgery before using the other one. If they change their mind then they should likewise need to redo bottom surgery before using their old bathroom again. There's no need for entirely separate transgender bathrooms, just use the one that is based on the parts or appearance.
Fist off, I find it a little extreme to actually legally force people to go to their respective bathrooms.
But nonetheless, going with this idea i believe it should be based on what the person looks like. If a male transgender wants to be a female and also mostly appears like one, the he (she) should go tho the women's room. This doesn't mean that the person has to have undergone any surgery yet. So if the man in this example dresses like a women, puts on make-up, probably a wig, then he should also go to the women's room. If this man then dresses as a man on an other day, he would have to go to the man's room, obviously. This, of course also applies to women, who want to be a man.
In my opinion this is the best solution, because , I believe, that is what the transgender wants, and it would also be less distracting for the other people, than to have a person who looks completely female to go to the men's room, or vice versa. Especially if the person has already undergone a lot of surgery and hormone therapy and thus looks very much like the opposite sex (except maybe for the genitalia, which are impossible to see, as long as the person keeps on his/her clothes).
Putting aside questions of identity, dysphoria and the psychological pain and harm for many trans* people associated with doing things coded to their birth gender-assignment - since that's probably too abstract for the majority of cis people (non-trans*) who don't experience dysphoria in their everyday lives to understand - the more pressing point is that the average trans* person (particularly of the assigned-male-at-birth variety, given the violent culture associated with the performance of conventional masculinity) is likely to be physically unsafe in a bathroom associated with their birth-assigned gender. The average cis man should try presenting in female-coded ways and see how many times he gets beaten up when he uses the men's room.
Another thing that disturbs me is that someone even thought this was up for debate. No, trans* people's rights aren't up for debate any more than yours are.
Even going to the toilets, are you more likely to see someone's genitals or their face? If they're going to the ladies then there are stalls so no one really even has the opportunity to see what's under the clothing, and if they're going to the gents then do you really look at their bits? I mean, do you honestly stare at the penises of other men when you go for a wee? Because if you do that honestly makes me more uncomfortable than the genitals of the guy next to me, which I am most certainly not looking at.
On the other hand, if you saw someone who appeared to be a man going into the women's room, would your first thought be a)call security or b) ponder their anatomy? A lot of transgender people pass as the sex they identify with long before they have any surgery at all. Lots of FTMs don't ever even have bottom surgery because the results aren't really that great and it's quite easy to pass in the vast majority of situations without it. Do you really want all those muscular bearded men using the women's toilets, just because they have a vagina that no one will see? Would you want to be consigned to the ladies too if you lost your penis in a tragic hedge trimming accident? Either way the women in there would likely freak out a bit if they caught what they perceived as a guy walking in.
You go into the dressing room for one of 2 reasons; either you're in there to use the restroom or to change your attire. There's really no reason to assess genitalia there. This wouldn't even be a concern in the first place without gender specific discrimination. This also excludes neutered and/or intersex people.
Of course not, you could only support this viewpoint if arguing from a place of ignorance. As others have stated, many trans people won't "look trans", and so it would cause far greater distress to everyone involved to force people into using the toilets on the basis of their genital arrangement. Think about it: how would you enforce such a rule without being massively inconsistent and invading people's privacy?
That aside, I don't want this argument to be based on the comfort of cis people e.g. "do you really want bearded men sharing the same pissing place as you?" because that's not what its about, its the fact that toilets are already an unsafe space for trans people due to this kind of ignorance, and that this is something which needs to be overcome: we should all have the right to pee in peace. This aside, the fact is that improvements in infrastructure can also be made in order to make toilets/ changing rooms safer spaces for everyone, and this is what we should be striving for. More gender neutral facilities (for those who wish to use them), better cubicles, and thinking about issues such as the frequency of foot traffic past entrances to toilets can all help to make the act of emptying one's bowls a more pleasant expereince for all; and not just people who have an irrational urge to police other's genitalia.
I am not going to share a restroom with a man like Buck Angel just because he has an innie. Nor is it fair to anyone to make someone use the men's restroom when they appear to be overtly female, regardless of what is between someone's legs. I'm not sure how it works where other people are from, but where I live, if you're parading your genitals around the restroom, you're doing it wrong.