Eh. As a straight cis ally, I've seriously thought about this question... Does it make you transphobic not to be attracted to a transgender person? I think it really comes down to whether you've decided you are not attracted to them BECAUSE they're a transgender person. It's an important distinction.
Here are a few more questions to ask yourself: What is it specifically you're not attracted to? Is it the genitalia or is it the fact that they *used* to be a different gender? If it's the genitalia then does that mean you would date a transgender person if they had their bottom surgery? Would you date someone who has had their bottom surgery but does not conform to socially acceptable norms for their new gender? Would you date a non-transgender person who doesn't conform to socially acceptable norms for their gender as assigned at birth?
There's no real easy answer to any of these questions, and each person needs to decide for themselves. For me, I've come to the conclusion that I'm attracted to who I'm attracted to, and if the person happens to be transgender, then we'd figure it out together. However, I say this knowing full well that it isn't an issue I'd ever been confronted with, as I'm already married.