Yes, we recognize age limits. We've also recognized gender limits by not allowing same-sex marriage. That's the norm that some want to change.
Age limitations are base on practical limitations. While there are exceptions that you brought up, the majority of teenagers really is not capable of making rational decisions. The real practical underpinning here is to protect them. We can demonstrate that absence of this limitations will result in harm to someone.
What is the hard practical reason behind the non-acceptance of homosexuals marrying? Who and how is harmed?
Precisely. It is based on our own existing norms, and those norms have dictated that gay marriage is not acceptable. Over two-thirds of the population agree with that statement, which I would argue makes the heterosexual-marriage-only position the norm.
This is not a very good argument for anything. Large portions of the populace at one time agreed on stuff that was later found to be wrong: flat Earth, slavery, miscegenation.
I keep talking about adapting something from other cultures because the gay rights movement is trying to adapt something from a culture other than ours
That's an interesting point of view, but it really is your invention. What basis do you have for this?
I'm throwing out examples of acceptable norms from other cultures because I'd like you or someone else to say why those are bad but gay marriage is good. If we can't say why doing X is not an acceptable change, but gay marriage is an acceptable change, then I can see no reason not to adapt X if we're going to accept gay marriage. If we're going to have significant societal change, get it over with all at once, because we'll be revisiting all of this within a decade or two as other groups use the legal establishment of gay marriage as precedent for what those groups want (more than likely Muslims, since their populations in western countries are growing rapidly).
Well, your clever plan ain't gonna work. :) I'm not going to say anything of the sort, mostly because I think that your initial premise that this is something from outside of our culture is wrong.
There is no internal inconsistency unless you begin with the premise that gay marriage and heterosexual marriage are equal institutions, and it is only our existing law that is inconsistent. However, we have hundreds or even thousands of years of western norms saying that gay marriage is not acceptable, so there is no inconsistency.
I think you have to start one step before that. I think that the premise that marriage is an institution is, at this point in time, incorrect. For evidence, just look around. Marriage today appears to be a legal construct, a special type of contract registered and enforced by the state, in essence, a service. Various religious groups apply their own significance to it, but this does not cross the boundaries of the group and there is no reason why other people should be forced to attach the same significance to it that you do. The inconsistency is that the state provides this service to some citizens of legal age and not to others.
There is nothing to stop gay couples from having marriage ceremonies, and little to stop gays from getting changes in law recognizing partners' rights in financial matters. Gay marriage advocates want something quite more than that, and for reasons that go beyond ceremony or finance.
What would that be? Sounds real ominous.
Social norms that have lasted for centuries usually have some practical basis. Up until not that long ago, a young woman becoming pregnant before marriage was something that was kept private, not something to celebrate. The change in that norm--along with several other factors--contributed to an increase in out-of-wedlock births, which in turn leads to all sorts of social problems. It's in society's interest for children to have two parents.
It's also in society's interest --until proven otherwise--that the two parents be of the opposite sex. I can't recall having read of any culture where homosexual marriage is commonplace, which leads me to believe that other cultures past and present have decided the same thing.
Considering that these days marriage is not reserved solely for raising children this is also not much of an argument. How is a gay couple different from an infertile or consciously "childfree" couple in this respect?
We've been playing with changing all sorts of social norms over the past 50 years, and the results haven't always worked well. Keith Richards notwithstanding, junkies tend to not live very long, and thus heroin use isn't celebrated
This isn't really comparable for the reason I mentioned above. There is clear demonstrable harm in heroin use that is absent in the gay marriage.