It doesn't quite make sense to me
I'm sure there a whole lot that doesn't make sense to you. Maybe if you would bother to educate yourself about subjects you intend to critisize beforehand then you wouldn't have so much trouble making sense of things.
why you would be opposed to teaching creationism in school, while simultaneously want to teach evolution
I'm not opposed to teaching creationism in schools, I'm opposed to teaching creationism in a science class as if actually science. It's not, and no matter what kind of spin you put on it will make it so. Creationism is not science and as such is not "owed", nor does it deserve a seat at the table in any science classroom.
Creationism, and Intelligent Design for that matter, have no grounding in science whatsoever, neither position posseses a body-of-work of it's own, their entire strategy consisting of nothing more than poorly sourced, intellectually dishonest, and blatantly false attacks on strawnman versions of evolutionary theory that have no real world analog.
I have no problem with creationism being taught in an elective comparative religions course, provided it is taught alongside the creation myths of other religions, and not given special treatment, presented as if it were irrefutable fact, or used as a opportunity to preach to students, make converts, or malign and insult students of other faiths or no faith at all. Creationism, as you envision it, isn't an inclusive model favourable to several religions, but rather a judeo-christian-centric concept that, by it's very nature, excludes all non-biblical religions.
It is not the place of the public school system, and certainly not the government to support or endorse any religion or it's creation myth. The only reason you want creationism taught in public schools is to prop up your religion with the color of state authority, if that were not the case then so many of you people wouldn't balk at the thought of having the hindu, muslim, celtic, or countless other creation myths taught alongside your own. If you want your kids taught creationism then do it on your time and your dime.
It's just as equally unfair to teach the population something that a portion don't believe in.
The theory of evolution is not a matter of "belief". People don't believe in evolution, they accept the theory or they don't. There is a portion of the population that doesn't believe in heliocentric theory, should we teach that the earth revolves around the sun and that it does not? There is a portion of the population that do not believe that non-caucasians are actually human beings, should we also teach that as well?
Whatever any portion of the population "believes" or does not believe is irrelavent to a science classroom, science deals in evidence, not belief.
You should probably be opposed to both being taught in school.?
Why? One is a perfectly valid scientific theory backed by confirmable evidence and 100+ years of research and refinement, the other is a religiously based and religiously biased idea that relies on an internally inconsistent book of dubious authorship, neither allows for nor tolerates refinement of any sort, and posesses no evidence whatsoever and relies only on an argument from percieved authority.
The former complies to the scientific method in that it requires evidence before making any conclusions, makes predictions that can and have been confirmed, and is capable of modification from the introduction of new evidence. The latter does none of those things. It's only evidence is a book that is supposedly true because it says so, and all evidence that disagrees with that single source of dubious authority must be disregarded. It does not, and indeed cannot make predictions of it's own, relying instead on unsourced inferrence and interpretation. It allows for no modification in light of new evidence and where the evidence and the claim disagree the evidence must be discarded. In fact, where reality and creationism disagree creationism's response is that reality must be wrong because creationism does not allow for any flaw on it's part.
One of those is truly science and therefore belongs in a science class, and the other clearly is not science and has no place in a science class, not matter how many people "believe" in it.