An additional reason for some people is equality. Apply the laws equally.
It's like saying, "Two black people can't get married like two white people can; they can become domestic partners, though!" That notion is offensively unjust to a lot of people.
> By law, marriage is between man and a woman. That’s it.
This is not true in the US. My neighbors are the same sex and legally married. I know at least 3 same-sex married couples here and I don't even live in a big city.
But if you're OK with that kind of discrimination, then you wouldn't have a problem if the law said only gay people could get married--but don't worry because straight people can get a domestic partnership. Yes?
The problem with that is, of course, that we're not treating people equally under the law when we should.
Sure, it was the case that that particular kind of discrimination was allowed, but we stopped doing that. We've stopped doing a lot of things that were discriminatory for a lot of reasons over the centuries.
In my country, we're all Americans, and we should get the same marriage and domestic partnership rights, gay or straight.
That said, I would like to hear how two people of the same sex getting married has personally affected you in a negative way. Is it because you feel the institution is lessened by allowing same-sex marriage, and so indirectly so is your marriage? Or something else?
My friend, I fear you are a long way from the battlefront.
> Following your logic, if 10 “consenting adults” would like to form a “marriage”, we should embrace that as well?
You need to say something like "it's legal for 10 straight people to get married, but it's not legal for 10 gay people to get married." Then the extension would hold.
But saying that no people can marry 10 people regardless of whether or not they're gay or straight, that's still equal under the law.
It's like saying, "Two black people can't get married like two white people can; they can become domestic partners, though!" That notion is offensively unjust to a lot of people.