[aishah]

others have covered the difference between sexual orientation and gender identity.

as far as passing, i may be wrong, but here is my understanding...

it is based on other people's perceptions. most people automatically sort others into "male" and "female," almost always with the assumption that the person they are looking at is cisgender. if they cannot clearly gauge they might assume someone is trans* or otherwise gender non-conforming. passing as male means adopting/developing enough physical markers to be read as male by others some or all of the time. if you are trans* and pass most or all of the time, since most people assume others are cisgender, i imagine you'd have to out yourself in order to not be perceived as cis, if that makes any sense.

re: passing as straight...regarding sexual orientation, it's pretty much the same as with any other person of any sexual orientation passing as straight or not passing as straight.