It does not have to. Gay marriage is a good thing, theres not one proper reason against it. Not a single one.
well, couldn't resist any more. Here comes the wall o'text:
Yes, yes there is a proper arguement against gay marriage, and you probably agree with it. No, it isn't what you might think.
First of all, one must understand what "marriage" is. Marriage is two things. The first thing is the legal part. Marriage allows you to be able to pass your inheritance to your significant other, it allows them to visit you to the hospital, it allows you to buy the "family ticket" at the park and so on. It is NOT directly connected to ANY monetary benefits. These are connected to children exclusively, at least in Greece and as far as I know in the US too (else all would get married to get free cash).
The second part of the marriage is the whole religious thing. Yes, it exists. Marriage, in the western world (and gay marriage IS a western world issue) is directly connected to the christian church. Now, before you scream and -1, read on please. This is not the LEGAL part, however it is a deeply entrenched tradition.
The reason religious people are against gay marriage is because they think it will redicule their religious institutions which, I am sorry to inform you, are not very gay-friendly.
The reason homosexuals are not content with a "union" and want "marriage" is because they believe the word "union" degrades them as individuals and want full recognition.
So, here is my arguement against gay AND straight legal marriage: they don't need to exist.
Legal marriage does not give any benefit to society. A legal union is all that is needed and shouldn't even be due to sexual connection. Being able to pass inheritance or allow somebody to visit you in the hospital does NOT require any sexual connection. Most people don't know that marriage wasn't, in fact, connected at all to the state until the early 19th century and then it was passed to stop blacks from marrying whites. Yes, I am completely serious, look it up.
Every benefit attributed today to marriage (and not having kids, please seperate the two) can be easily transfered into a "union" contract that could be done between a man and a woman, a woman and a woman, a man and a man, a man and 2 women and so on. The religious people will not have a problem because it isn't "marriage". The homosexuals won't have a problem because...well, the religious people won't be "married" in the eyes of law and if somebody wants to go to a church/synagogues/ a nice bar in hawaii, bring some guy in a funny dress to say a few chants and call themselves married, it is their own god damn wish and nobody can and should stop them.
See? I told you that you would probably agree :3
edit: and of course, today's union IS lesser. For example, they can't adopt children. This would obviously not be an issue in the actual "union" I am proposing.