Is feminism necessarily pro-transgender in its ideology? Many, and maybe most, feminisms are clearly supportive and welcome trans*folk into their ranks. However, some branches of feminism take the opposite approach, arguing that the transgendered, while having a right to be whatever they want to be, are nevertheless NOT welcomed into women’s spaces.
Over on feh-muh-nist, you’ll find a short essay detailing this position — the comments also lay out the scope of the argument.
Feminist Reprise has a resource page on the question of the relationship between feminism and transgenderism — espousing sentiments ranging from “trans-is-not-a-real-phenomenon” (a la Bindell or Raymond) to feminist separatism.
Finally, there used to be an entire website called Questioning Transgender Politics, which was filled with essays detailing this position, but the website has been taken down. Please see the essay Money, Mouth, over on Questioning Transphobia, for a summary of this website’s positions and possible rebuttals.