I'm somewhat ambivalent about this, mainly due to the issue of what marriage actually means these days. I'm all for homosexual couples being allowed to be together, but the current issue is somewhat removed from this, which is kind of sad.
When I think of marriage, I don't necessarily think of it being something that's entirely good. Not that it isn't a beautiful thing at times, but it has been abused to no end. I also think of whether this kind of status actually matters. It is of course an artificial construct and so has no inherent meaning, so I feel that if the world were exactly the same as it is now, except without marriage having introduced, homosexuals and their proponents would be exactly where they want to be.
This is a farfetched scenario, but it's more for illustrative purposes. Marriage is somewhat erroneously placed on a pedestal, despite its abuse and subsequent weakened meaning. This glorification, in somewhat crude terms, makes gay couples think 'I want that', when it wouldn't necessarily be an improvement on their life or relationship. It feels like it's more that they want it just because others have it.
That said, there's still no point in going out of our way to prevent it, but there's still no point in going out of our way to introduce it. Obviously religion is heavily tied into it, and the Bible isn't being rewritten anytime soon, so legalising it's just going to be more trouble than it's worth. I feel this is the only deterrent; if religion weren't an issue, there wouldn't be a legitimate reason to not introduce gay marriage at this point.
The religious connotations contradict the motive for gay marriage to an extent, since I would imagine anyone (or at least a majority of people) who would enter into a gay marriage would not be religious, and would therefore have no interest in religious ceremonies. A civil union seems to be what is actually sought, but I guess it doesn't have the same ring to it.
This is more just me, but I don't feel that external recognition is important in a relationship. I understand that feeling like you have to (and actually having to) hide your relationship can be difficult. But our society is mostly past that point now; it's only really a legal barrier rather than a social barrier, at least from my experience. If a guy can hold a guy's hand in public without anyone batting an eye, then that's about as much acceptance as you'd need. Well, maybe I speak for myself. I care more about internal recognition of love. External recognition is just a bonus in my eyes. I understand not everyone feels that way, but if internal validation is lacking then I don't know how many problems getting married will solve.
If I love someone, I'd be happy to live with them, and have, or adopt, children with them. Marriage just happens to exist, so it shouldn't really affect something that is so innate as love. If someone's last thought, on their deathbed, was 'I wish we got married', then that would be a little depressing. I would question how much their relationship really meant. This goes for a member of any couple.
But, for sure, the situation could be improved upon. There are many issues surrounding it that are a long way away from repair (e.g. religious involvement), but there are also many other issues in this world that might be more deserving of our attention.