Your point about fantasy - yes, I agree with that.
As for your question "why would your clothing make you change your sexual desires", the answer simply has to be that it doesn't. I do however think that for some sexual desires can influence their choice in clothing. To use a (admittedly stereotypical) example, butch lesbians. They incorporate a look that reflects their sexuality. Their look not influencing their sexuality.
In regards to cd's who wonder or fantasize about having sex with a man, for many this seems to be more about a desire to affirm their female persona rather than a desire to actually have sex with a man. It's a self-reflective fantasy all said and done.
As for your comment about not understanding why people think being a woman is always a passive sexual activity, well, in terms of conventional sex, that is exactly what it is for women. Biology dictates that. This passive/active dichotomy however lends itself to homosexual relations. A giver or a taker. Top or bottom. These are merely preferences, and despite the connotations aligned to the words passive and active, it is not meant to imply weak or strong. Or be detrimental to one over the other in any way.
Of course, you are correct in that people, regardless of their gender (or rather biological sex), can adopt either role. For women (whether they are with men or woman), should they take on a more active (as opposed to passive) role, it means that they are taking on a more masculine (as opposed to feminine) role in the engagement. By the very definition of what the words masculine and feminine mean.