This is also a concern of mine, but in the end a man must simply take the attitude that it is just one of the risks that go with being sexually active. That said, I do try and make a "tranny appraisal", and if it seems more likely than not, I bail out. I will say that not too long ago I did get fellatio from a possible tranny (she did have female plumbing), and I must say that it was one of the best blow j0bs that I have ever had, just saying.
I think my tranny-dar is more sensitive than most. I remember a going-away get-together for a colleague who had some woman that appeared to me to have tranny characteristics, including wearing a turtleneck on a subtropical summer evening (to hide the Adam's Apple laryngectomy scar?
) and I asked my other colleagues if they thought she was was a tranny, and everyone thought I was crazy.
I think the main reason why it seems so prevalent these days is that trannies have 2 of the most important qualities for casual sex: fitness and motivation. Most REAL women these days are fat pigs, and the only ones that are not tend to be ectomorphic, which is something that a slight-figured and motivated man-to-woman can easily do. And I would hink that, of course, it must be of the highest self-satisfaction of a tranny for him-her to be able to trick a horny heterosexual man.