[vxNsr]

I always thought the argument was that it's about genitalia more than anything else... aside from the few super crazy people most I spoke with just don't think it makes sense for someone with a penis to be using a woman's bathroom and vice versa. Especially if they don't look like the socially, generally, and visually understood gender the bathroom was meant for. Meaning that if someone who most people would identify as a man walks into a woman's bathroom that might make some woman feel uncomfortable, and men had the same reaction, they would feel uncomfortable if someone who visually looks very similar to a woman walked into a men's restroom they would feel pressured very uncomfortable.

[niftich]

I highly doubt this, since genitals only really determine if you can use urinals effectively, and non-urinal toilets are also found in the same bathroom as one that has a urinal, as they're needed for other stuff. Therefore, there is no need to legislate genital-to-bathroom mapping (regardless of your political and/or social beliefs) as the capabilities of your body will sort it out.

On the other hand I can better understand the argument that people's appearance causes us to make certain assumptions about whether they are of a particular gender, which causes us to make certain assumptions about their sex, and that presence of the opposite binary sex in bathrooms has been historically considered taboo, or unwelcome, or creepy, or [insert visceral reaction here]. This is real meat of the debate, because it pits people's self-identification against others' profiling, and one group of people's right to privacy to a different group of people's right to privacy.