"Colon cowboy" is not technically accurate - unless you fuck her in the ass. She is post-op, so she has a 'pussy', albeit a manufactured one.
But I get it. Most guys have a mental aversion to being intimate with someone who has male DNA, even if they look and act completely like a female. I envision a future where you perform a quick DNA scan before proceeding with the encounter...
If this were not an SL thread, it could almost get philosophical!
I guess I concur with Joe Buck. Based on the passions expressed, it is best to fully disclose up front to avoid the possibility of someone getting hurt. I believe most TS do in fact disclose this. But I can see how someone who has had the 'change' and always thought of herself as 'female' might lose sight of the reason for disclosure.