But this line of yours is logically unsound. (The latter half, that is.) There is nothing physically unhealthy about a brother and a sister, mother and son, etc. getting married. The health issues come up only if they reproduce. That could be separately legislated: people who are too closely related can be forbidden from having children together (naturally). And lets face it, many people marry without any intention of having kids.
So if you're going to argue on a strict libertarian basis, you can't justifiably forbid incestuous marriage. (I do not agree with your position myself---I'm just pointing out the arbitrariness of where you draw the line.)