Perhaps a troll , but if you're actually interested in this it's a very interesting and complex issue of scholarly and political debate. Somewhat recently a prominent feminist journal published an article that considered the question of "trans-racial" identity and rights in apposition with commonly accepted [among scholars] frameworks of transgender identity and rights. Their argument, which i think was rich and not unconvincing, was that the same frameworks that support transgender individuals should also support transracial individuals.
The article did receive severe criticism and widespread backlash; I think many feel that the arguments advanced against the article were conceived in bad faith, and that the reaction should not have been so damning.
At any rate, to really seriously consider the question one has to think critically and deeply about the social categories of race and gender as they've played out historically, and to think about how different ideologies of race and gender interact to support and/or undercut one another. My personal feelings are that because trans rights are only just getting traction outside of activist and scholarly circles, and the idea of "transracial" identities is perceived as utterly laughable and extreme to most of the public, many politically minded people try to refute the comparison so as to better preserve the legitimacy of the fight for (transgender) rights. Indeed, many social conservatives attempt to argue against trans rights by making similar statements that it can count on the public perceiving as absurd ("what's next? Even though I'm white I'm actually a Mexican!?" etc); no one invested in gender liberation wants to inadvertently give these people more fodder.
There are also important and critical questions about social privilege that attend these issues; and it is interesting to compare some of the points commonly made against transracialism in left-leaning circles to similar arguments made against the acceptance of transgender individuals (See Trans-Exclusionary Radical Feminists, etc). At any rate, these really are complex issues and if you want to dive in I would think you'd want to start by reading some of the important works in feminist , queer, and transgender theory as well as critical race theory and race relations; and then attempt to synthesize or compare the viewpoints yourself or find serious work that broaches these questions.
The sad thing is that most will be content to accept their gut reactions of what is "obviously" absurd to dismiss and attempt at critical inquiry into these issues, preferring to reassert something called "common sense" without any thought as to how what masquerades as "common sense" is actually a complicated knot of ideologies impressed upon us by the [often oppressive] forces of the social, political, and cultural world(s). At any rate, I hope my sketch of some of the important aspects of these debates is helpful to those interested in critical reflection on these issues.