There is no such thing as gay. There is only normal human sexual expression.
OK, you obviously need this broken down into simple steps.
Step 1: Your genes define who you are biologically. Sex is hereditary. If your parents did not have it then neither will you.
Step 2: Your genes do not define who you are sociologically. The idea that there are gay genes is a political idea. It is born not of any fact but rather from those who have the specific agenda that gay is identical to black or Asian or whatever. Because of this theory, if one is gay, then one deserves to have rights which are at least as powerful as those who can't think of a reason why they should have more rights than others.
Step 3: Your genes tell you that having orgasms is a good idea. Past that, the idea that there are genes for liking men's butt holes makes about as much sense as there being genes for liking blow jobs and genes for compulsive beating off.
Step 4: There is no such thing as “gay”. There is such a thing as normal human sexual expression. Some people like some things. Other people like other things. Still other people think that something else is hot.
Step 5: if there is any genetic aspect that is relevant, it will be that we all like to have orgasms. Cumming is fun. Most of us like to cum and those who don't tend to not reproduce.
Step 6: Some people are into MOTSS. Other people are into MOTOS. There is, in point of fact a continuum of normal sexual expression. If someone is 90% straight, then the odds of their getting it on with the same sex are minimal. The same but opposite would be true for someone who is 90% gay.
Step 6a: You are not locked into a place on the continuum at birth. There are plenty of people out there who have large families and decide later in life that they want to do people who are the same sex. If they find someone else they can work with, then I am happy for them.
Having gone this far, I will now tell you where I stand on gay marriage.
Let them get married.
The usual argument seems to be about families. Well, my family is not so fragile that two dudes a thousand miles away have any bearing on what I have going on. If my family was that fragile, it would reflect on me and not on the two guys next door.
Then the next argument tends to go into the idea of the “rights and privileges of marriages”.
Every time that I hear that one, I am like “huh? What?”
Sure, there are tax credits and shared medical insurance that go with marriage. Those are financial things and not relevant to human sexual expression.
I suppose that a gay couple could get with a lesbian couple and have two marriages. After that, anyone can fuck whomever they want to and they all get the tax credits. Granted, that would be a kind of separate but equal thing. We all know that that is balls stupid but it is a trivial way to manipulate the system.
Where this really matters is not about the rights and privileges. It is about the consequences. I fail to see how a gay relationship cannot go sour as easily as a straight one.
If two dudes want to hook up and not get married, well, then they get to have some fun. Ten years later, they have to figure out who gets custody of the french poolde and tell their respective insurance companies what the deal is (well that could happen if insurance companies were not behaving like financial leeches).
On the other hand, if the same two dudes got married, then when the deal goes bad, they still have to do the same stuff. The difference is that the paperwork is pretty much the same. If stuff is the same, then how is it different?
If the deal is that same sex marriage is all about the stuff and never about the costs, then I would have a problem. Shall we create a concept of a civil union? Sure, if that floats the boat for someone who wants it. Just don't make it all about what is good to have and never about the costs. That would be the same as carving out a special section of the law, which is not what I am hearing the gays asking for.