As I stated, I feel it SHOULD remain illegal solely because you are forcing a group of tax paying people to pay for something they do not morally support. Yes, there are other facets that tax payer dollars may go towards (wars that people don't agree with, but that goes under defense) and I think we shouldn't pay taxes period, but that's outside the argument. Basically, I am saying it should be illegal in a Libertarian society because by legalizing it you are making saying to the Ant-Gay Marriage "not only do you have to deal with it, but you are paying for this. Ha ha"
As for the incest scenario, if they are not having sex, they have no reason to even entertain the concept of marriage, so I am not sure why that would matter.
To propose that we not pay taxes is essentially to propose that we don't have a government. Taxes represent, if you will pardon my use of a somewhat strained analogy, a symbiotic relationship between the government and its people. The people are served and in some cases protected by the government (although with things like the patriot act, there are instances in which the government screws people over as well, but this is not the ideal condition and will hopefully soon be rectified), and the people, in turn, pay taxes in order to fund the government. Many politicians, particularly republicans of late, like to gain votes by promising to implement, and often actually implementing, tax cuts of some kind. This really does more harm than good. Without the revenue generated by taxpayer money, the government would have an incredibly difficult time financing much of anything, including public education, healthcare, etc.
And then there's the inherent flaw in the initial argument, this being that people shouldn't have to pay taxes for things they don't support. This is in essence the same logic many use when justifying cutting taxes from education funds: If they don't have kids, why should they pay for public education? The fact of the matter is, if we want to have a government, a system of taxes is practically a given, unless somehow lottery programs and impossibly lucrative charities can be expected to keep an organization as enormous and expensive as a government together, which they really can't. If we argue that the government should stop any program that any group of tax-paying citizens disagrees with, then we are effectively arguing that the government should be dissolved, or at least crippled to the point where it is incapable of doing much of anything.
Therefore, my previous statement stands. In an ideal system of laws, no right should be denied without a pragmatic societal reason. Making homosexual marriage illegal can easily be seen as denying a group of people a right of some kind. As of yet, I have not been presented with anything like a logical reason as to why it is necessary to do so.
What I have heard are the following:
1. Arguments involving morality and/or scripture, which are obviously inadmissible because the goal of our system of laws is not to adhere to scripture or the particular morality of any given ideological group.
2. Hypotheticals involving the proverbial floodgates of other sorts of marriage being declared legal, including, but perhaps not limited to:
-Marriage between humans and nonhumans, which is obviously a flawed comparison, as consent could not be explicitly stated.
-Marriage between adults and children, also illogical as it ignores once again the problem of consent, which legally cannot be given by a minor, for the very practical reason that by and large most children lack the maturity and forethought to fully understand such a decision.
-Marriage between family, discussed just now.
-Marriage between over two people, which is already legal in some respects ("open" marriages) and regardless is far too abusable to make any practical sense from a legal standpoint (Large numbers of people marrying in order to share the legal benefits).
3. Bizarre end-world theories, which make the fallacious assumption that legalizing gay marriage would cause homosexuality to propogate like some sort of virus, somehow ending reproduction as we know it, and thus ending the human race.
4. The argument which you've presented regarding taxes, which I've already covered.
I would call none of these practical reasons to thusly deny rights to homosexuals, and at this point, I am highly doubting that any argument could be made that doesn't rely chiefly on emotional charge.