The Supreme Court CAN move this decsion to the states though, by overturning Roe v. Wade. There are 2-3 judges who would be potentially stepping down, and you could be rest assured that Palin/McCain (or McCain/Palin) would recommend judges that would want to overturn this decision. If you happen to live in a conservative state, and Roe v. Wade is overturned, that DOES imply that you might be denied this right. Same holds true with constitutional amendments that ban gay marriage. While you say that repubs don't want to legislate your personal beliefs, that doesn't hold up if you look at this way.
OK, first things first, NO State has ever had a law on the books that would prevent a woman from terminating a pregnancy that is the result of rape or incest, and as a matter of fact, the procedure to do so was ROUTINELY done, at the hospital, when the victim was brought in even in the 40's. It's called a "D and C", or Dilation and Curettage, which is an OB/Gyn procedure where the uterus is essentially scraped out, removing any "foreign matter" (including the fetus) from the womb, and was the standard way of dealing with a rape or incest victim before the modern "morning after" pills were available. Today, all that's needed are the "morning after pills" which are administered to all rape victims as soon as they're brought into the ER. So, this whole "the States will prevent rape and incest victims from having abortions" is utter rubbish on it's face, and the only way you're "looking" at it is through the lens of fear and ignorance. I also suspect (but cannot confirm, so if I'm mistaken, forgive me) that your real agenda is to keep abortion legal at all costs, and if this is the case, then your entire "rape and incest" argument is nothing more than a Red Herring attempt to distract from the real debate.
Secondly, there's already a Federal "ban" on gay marriage, and the States are already able to decide for themselves whether or not to ban gay marriage. As has been pointed out already in previous threads, as far as the government is concerned, marriage is a contract, and since the states already have the legal Right to determine what contracts are, and are not, permissible within their borders, it's a moot point.