The reason to allow gay marriage is because it's fair.
The theory of Human Rights basically dictates that you can do what you like as long as you don't hurt others. Brother and sister marrying is not allowed because any offspring from such a relationship is at risk, because for some biological reason bad things happen when children are born to such close relatives.
Also, when you marry you become related. You're already related as brother and sister. Therefore you don't need to get married to be related. Things like inheritance aren't a problem between siblings whereas they are for those who can't get married.
For example, a gay couple decided that the older one (by a year or two) would adopt the younger one in order to get the security they could not get because gay marriage had been unavailable.
Gay couple, one of whom adopted the other, has adoption vacated now that they can marry
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Gay couple, one of whom adopted the other, has adoption vacated now that they can marry"
"So, in 2000, Novak adopted MacArthur – two years his junior – as his son. The move reduced their inheritance tax liability and gave them hospital visitation and other legal rights then unavailable to same-sex couples."
Inheritance tax is a massive issue.
How Marriage Inequality Prompts Gay Partners to Adopt One Another
"The couple told ABC News that they were primarily concerned about Pennsylvania’s inheritance tax, which could make one partner liable for a 15 percent tax on the estate (as opposed to 4 percent if they pushed ahead with adoption). "
An Overview of Federal Rights Granted to Married Couples | Human Rights Campaign
"There are 1,138 benefits, rights and protections provided on the basis of marital status in Federal law."
Wait a minute, I thought you said marriage was necessary because of government benefits; that we shouldn't be allowed to deny any people those benefits.
No, you are not "related" once you are married. You are simply two different people that took a religious vow or obtained a government sanction.
So you have marriage standards and limitations but others shouldn't? How is that right? If this is all about government benefits as you say, then no two people should be forbidden that right as you call it. For that matter, several people shouldn't be forbidden those benefits.
Brother and sister can have kids if they get married. Well, why can't brother and sister have kids without being married? If that's the problem, then it okay that father and son get married, or sister and sister? You're okay with that, right?
It seems to me you are a hypocrite. Most of society wanted to limit marriage to one man and one woman, but those standards are not acceptable to you and others. You have no grounds to expand those standards that shouldn't apply to everybody else.
I'm not saying it all makes sense. It would be better if the govt stayed out of marriage altogether.
But marriage is the linking of two people. But brother and sister are already linked. You are linked. Clearly. Women in many European societies take on the surname of their partner and are considered by the state to be linked.
A brother and sister are "simply two different people" who happen to have the same parents.
Is it right? It's not necessarily fair that sister and brother should be kept apart. But that's the way of the world and it should be something that is encouraged for logical reasons. However, like I said, gay couples who couldn't marry ended up being adopted by the other in order to gain most of the important benefits, so, brother and sister, father and son, already have many of those benefits.
Brother and sister can have kids without being married. However it's a matter of making sure that society sends the message that it is not okay for brother and sister to be together in such a relationship. Again, its biology, not necessarily fair.
It seems I'm a hypocrite huh? Well I'm sure you're one too.
You're looking for holes in an argument which will inherently have holes. Why? Because there are various parts to it that aren't necessarily compatible. However, like I've said, I'd prefer it if the govt stayed out of marriage and churches or institutions can decide who is married under their own house and nothing else matters. No benefits, not privileges.
If people want someone to inherit, then they merely write a legal document to say so.
However.... what we're talking about, and which you're trying to push, is what you might call a half way house.
If the govt is going to be a part of marriage, then it needs to abide by the 14th Amendment equality of the laws. Now, you say brother and sister should be able to marry, but under the law brother and sister already get lower inheritance tax and many of the other benefits. So.... is it unfair?
Society makes some things illegal. Murder is illegal. We discourage doing it by making laws. Brother and sister marrying is also discouraged for reasons I have said. It's not fair. It's life. Get over it. If you want that to be hypocritical, then so be it, I don't care.
However, which is better, fighting for the govt out of marriage and not getting anywhere, or having gay people able to marry and get the benefits and live their life as they so wish as the 14th Amendment and Bill of Rights dictate?