YES YES YES YES YES
Of course... It's still not necessarily *safe* for a trans woman to keep her transgender status private, but it is absolutely, 100% her valid right and choice to keep it private if she decides to take that risk.
The onus is on the people that have a problem with this, not on the trans person. If you have cissexist issues with having sex with transgender people, it is *your* responsibility to make sure that you're sleeping with cisgender people. Your bigotry and attitudes about the "reality" of transgender genders is *your* problem, not ours.
I've heard some people claim that some people just have a "preference" for cisgender people. Fine. But the racist analogy holds true again. A person that has a preference for white people... fine... But consider a person that is attracted to a person that appears white to the point that they sleep with the person or want to... and later find out that the person is descended from people of color. If they "lose interest" because of that person's history, that person is, in my book, a racist. Period.
Same for someone that loses interest in someone after finding out they're trans. That's cissexism, plain and simple.
(For those of you not up on the vocab, "cissexism" describes prejudice against transgender people, especially related to the validity of their genders. It is much the same as "racism" is used to describe prejudice against people of color, or "heterosexism" is used to describe prejudice against lesbians, gays, and bisexuals.)