In a white racist society, is a woman with partial black ancestry ethically in the wrong (not just potentially unwise for her own safety or anything like that) if she has casual sex with a white man, who may be racist, without mentioning her ancestry, in the chance that he will feel violated and deceived if he finds out?
After spending way too much time trolling the horrid site known as Stormfront, I believe that racists (particularly white racists) will always make sure that the person of interest that they wish to have a sexual relationship with will have white genes many generations back. No "ifs" and no "buts". So, the scenario you give, is probably unlikely. But, it depends on what your vision of a white racist is like and what my vision of a white racist is like.
I also asked you earlier if racism is an appropriate metaphor for this situation. Do you think it is an appropriate metaphor?
And, when you say "a woman with partial black ancestry"....do you mean a cis-woman or trans-woman? I just want to be sure. If a cis-woman didn't mention her black ancestry to the white racist - that would be funny. Why? Because that is not as serious as the situation of Tom and Shirley. Skin color is skin deep. This, on the other hand, is much deeper than that and more complex. I believe (and please, it's just my belief, don't hang me) using race-issues as a metaphor for this event of the OP is downplaying the seriousness of this issue of Tom and Shirley.