*dons firefighter gear* Ok, let me jump in with a quick opinion:
1. Marriage is mainly a social privilege between two willing people. Most usually, as it has been throughout history, that means a man and a woman. However, the most important thing about marriage is this: it brings different treatment both in tangible points prescribed by the law, in terms of tax benefits, medical care, and in intangible points such as social status, respect, (alleged) health benefits, stability and etc.
2. It is not fair to deny these benefits to any two willing people because they are of wrong sexual orientation; to do so would be discrimination. If the two people in question were (some other trait, say Jewish or Black, or White), it would clearly be illegal to exclude them from the right to sign the contract and get those same benefits - some would say unconstitutional.
2A. Sure, if we view marriage as a moral category instead of a contract (such as many religious institutions do), we might say it is between a man and a woman and no others. Fair enough! We can call it Marriage (TM). But we still cannot discriminate against other people - that would be illegal, as said above. Even if they're not called Marriage (TM) (this could be determined in court, i.e. is the term "marriage" copyrighted by anyone), their unions need to have the same privilege. Some might call it a "Civil Union", some might call it a "Marriage", or "Marriage 2.0". Personally, I don't care, but their rights need to be equal, otherwise it's discrimination.
3. The above two points were completely unrelated to the subject of kids. So, we've established that this union/marriage is the right of all people. But what about the kids? Homosexual unions/marriages cannot naturally produce children. Should they have the right to pay for and use a surrogate mother or a sperm donor for in vitro? Should they be able to adopt? Those are all things that people in marriages normally do, but these ones can't because of their biological constraints. Now, this is where the debate gets interesting:
3A. Is having children a basic human right? I say not. It is not an existential issue, or even close to it. There is no reason for the state to fund homosexual couples' visits to the clinic, or provide them with a selection of children to adopt. It might, but it is not their basic right. If they get it, it is merely the state being generous. Personally, I say that it is their (the homosexual couples) business as long as they do it on their own and with their own money.
No one should restrict them, so as long as the state has no obligation to pay for it, all is fine. If the state paid for it, then it should also pay for my visits to hot young prostitutes when I get old and unable to get young women, an example of another completely non-existential issue that I had the choice to resolve myself.
3B. But what about the fate of the children? What will happen to children who grow up in homosexual unions? Will it harm them in some way, so we should ban homosexual couples from artificially having or adopting children? Tricky question, no one has produced convincing proof that it would harm the children yet.
Personally, I believe that it might produce some harm - not too serious, but statistically significant anyway. Growing up with two parents and a role model of each gender is important for a child's development. If one of them is lacking, there could be consequences (I don't think it would mean that the child would be homosexual or anything - that's mainly genetics - but growing up pussy-whipped, without game (if male), or with attraction to jerks and father figures(if female)? Maybe)
However, our society already allows for no-fault divorce, single mothers (the kind who were pumped 'n' dumped by their own wishes) and all kinds of horrible stuff, that has been proven to be devastating to children. Sure, some children from single parent homes (mainly single mother, but let's not be picky) do grow up into normal, successful and well-adjusted people, but a very significant part does not. There was this famous image about Feminism = Bullshit floating around that pulled data from census, criminal records and etc. and found something like 10 times higher rates (10 times, 1000%!!!) or behavioral disorders, 3 times higher chance of suicide, 7 times higher chance of dropping out of school, 5 times higher chance of committing a rape, 20 times higher chance of depression, 2 times higher chance of divorce or teen pregnancy, etc. All of these are clearly disastrous effects caused by modern feminism and one of the reasons why I despise it.
But if the society and laws are fine with allowing divorce and pump 'n' dump single mothers, which have been proven to devastate children's lives, it would be unfair to ban homosexual unions from having children. There is no way that they can have worse effects than the aforementioned single motherhood cases, and there is a good chance that they might be just as good for the children as a normal two-parent heterosexual family would be, or perhaps a bit worse but certainly not worse than what the society already gleefully allows.
So yes to homosexual marriage and adoption of children*. I'm not a fan of it by any means, but to me it seems the only reasonable solution.
* = on the condition that no public funds are involved