nice points anosmic!
I'm not sure whether you're playing devils advocate (sorry for the pun), or whether you're defending the christian/conservative position, assuming you're christian. but that's not relevant.
in re adoption, I don't think we can reasonably demand optimality really. Sure it's debatable whether homosexual couples are going to be as suitable as heterosexual ones. But in the grand scheme of things, aren't there much more suitable and more predictive factors to screen for suitability as adoptive parents than whether the two parents are of the opposite sex? I mean, isn't eduaction or income or mental stability more probative? If we're going to discriminate on the basis of providing the best parents for adoption, sexuality is just not a very good criteria. Furthermore, I think if the homosexual couple is well suited except for that fact, then it's going to be a huge improvement for the child over orphanage either in US state sponsored care or asia/africa/south america where they are likely to get sold into prostitution or die of hunger.
in re marriage, I just can't see how allowing homosexual marriage is somehow devaluing heterosexual marriages. I mean isn't it a bit rich for the man/woman pairings to say "man, I don't want my relationship to even be compared to yours!"
But if you want to argue on a semantic level, that marriage has been culturally defined as monogamous union between a man and woman, just like a royal flush is culturally defined as the best poker hand, I would have to agree. I mean, if the accepted meaning of marriage along the sands of time really is what you suggest (and on this point I really have no idea whether "marriage" has mostly/wholly meant christian marriage in the past) then you'd be right: nobody has the right to force us to change a long standing cultural definition of marriage.
But lets now make the assumption that gay unions, whatever you want to call them, ought to get the same institutional/legal protections are traditional ones because anything else is tantamount to an untenable state sponsored discrimination (I'm reading that in between the lines of what you've argued).
The alternatives would be to legislate for "civil unions" or whatever else to call them, and replace all statutory provisions of marriage with "marriage or civil union". The other would be to define "civil unions" as being legislatively identical to marriage. (The third would be expand the definition of marriage, for which I can accept your cogent argument against). I think either of those proposals is what gays want anyway. I'm not gay, but I don't think they care that much whether their partnership is called a marriage or not, so long as the state sponsored systematic discrimination is remedied
Finally on evolution, the universe as we know it is very, very, very old
