I'm okay with any theory being in a science textbook as long as there is some kind of scientific backing.
Evolution has some scientific backing. It should be in a science textbook. It's science, after all.
If someone can find some real scientific support for creationism, that's great. You can put that into the science textbook, too.
Until then, whether you believe in creationism, intelligent design, evolution, some kind of mixture of that, or something else entirely, you have to accept that only science should be in a science textbook.
You don't have to agree with the science. It is just a way of understanding the world, after all, but a science book should have science in it, and not have non-science.
As an analogy, it probably doesn't make a lot of sense to drop the teachings of Hinduism into a new revised copy of the Koran. The Koran is an Islamic text; the Hindu teachings really don't have much of a place there. Doesn't matter which one you believe to be correct, if any. It's just information existing in its proper context.
So please, Texas education people, it doesn't matter what you believe. It's all about putting things where they belong. You can believe whatever you want, I really don't care (unless you want to kill me or something, then there's a problem), but don't put non-science into a science book. It just doesn't belong.