According to the Physiology section of the Wikipedia article, Hutts are hermaphroditic and voluntarily adopt gender roles - it's just a matter of convention that adults are generally referred to using masculine pronouns:

Though traditionally referred to as males, all members of the species are hermaphroditic; personality type and gender roles are assumed voluntarily, or upon a viewer's deduction. Upon becoming pregnant, Hutts are referred to as female, although some Hutts do not accept this and choose to retain their masculine persona, whereas others alternate between male and female character. (Wikipedia)

Hutts were hermaphroditic, and as a result, gender was more of a case of a Hutt's deliberate decision or a viewer's deduction. Often, Hutts carrying children were referred to as female, though it was entirely up to the Hutt in question as to if the distinction was accepted or not. (Wookieepedia)

While it's entirely possible that transgenderism may exist in the Star Wars universe, because it's not a 'mainstream' topic in the conciousness of most Western writers, there are probably very few (or even no) TG characters in canon or 'dead tree' publications (i.e. the 'expanded universe')

If you're after TG in Sci-Fi, it's probably best not to look West to Hollywood but East to Japan and anime / manga series. Or do a category search for stories classified as Sci-Fi here :)