If human beings insist on "Marriage" being a thing at all then it should be for everyone. Frankly It seems to me to be little more than social engineering.
But i'm sick of this "Marriage is defined as between a man and a woman and has been for ages" bullshit. Is it? Because I'd like to see some ancient sources on that, hell I'd like to know where that appears in the bible because I don't remember it being in there.
I'd also like to know how you reconcile that overused political propaganda phrase with Polygamy and group marriage, both of which have existed for thousands of years but go somewhat against "A man and A woman"...*ahem*..."Definition". If you're going to be stupidly anal, and I mean that in the most eyerolling are-fucking-kidding-me way, about the "definition" of a word down to the letter then you're going to have to deal with things like Polygyny and Polyandry and how they didn't ruin "Marriage" for the past few millenia.
People like OP are not fine with Gay relationships, no matter how many times they say they are, and this is evident by their insistence that they remain second class citizens on the "Relationship" scale. They demand that Straight, monogamous people retain the "Highest" status of relationship for themselves and exclude all others from the sacred clubhouse that is marriage.
Or, to put it another way, OP is fine with gay relationships as long as straight relationships remain superior and have more privilege.