It has to do with the fantasy of it all. I think anyone who is sexually adventurous wants more from sex. Tgirls are exciting because they are different. They are uber-feminine which is what the initial attraction is but then when you know that there is more, its like a secret...that only you two share. It makes it that much more decadent and plays into the fantasy of it all.
I have of course been with Tgirls where the fantasy breaks and the reality of it hits you but Tgirls like Mariana or Angeles or Ana, they have the ability to transcend that reality and give us a world which is not available anywhere else...
I think the boobs, ass, cock all play into the fantasy...and all a man is left wondering is how could I perhaps experience a part of the world of this beautiful creature...