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.)
I don't think you've thought this all the way through.
Being legally married comes with a lot of baggage re: inheritance, insurance, credit ratings etc. Would you therefore, FORCE religious organizations to extend to "gay marrieds" the same insurance bennies they give to their heterosexual couples? Should the taxpayer have to pick up insurance for some gay person's "spouse?" You can't get your own blood PARENT picked up by your insurance bennies with the government..but some "lover" you met in a bathhouse two weeks ago and "married" is now supposed to be supplied with taxpayer financed heath bennies? Gee, the public is going to love that one.
And re: incest....just suppose that one of the two parties is STERILE...no chance of having "funny children" what's to prevent 60 year old dad and 40 year old daughter getting "married" if one or the other is sterile. You also forget that with parental permission, minors MAY get married. What if "Heather has two mommies" mommies both decide that 16 year old Heather should be allowed to contract "marriage" with HER 35-year-old lesbian lover? An "old style" Mormon decides he wants 18 wives? Why not...what are you doing pushing your morality on them.
It's an intellectually lazy posit that "you can't legislate morality" -- the hell you can't. We do it all the time.