"My point, dear Mafia, is that marriage has a real meaning, a real sanctity," - Agreed. It does. And that's why the LGB community wants access to it. Because it has meaning. They don't want to be fobbed off with something that doesn't have meaning, like "civil partnerships".
"And is not an accessory for people who, optimistically, believe appropriating the word through political pressure, will gain them more acceptance in society." - Appropriating the word? Saying that is to make the assumption that the word belongs to straight people in the first place. It doesn't. The word belongs to all of society, everyone knows married people, everyone acknowledges marriages a certain way. And you make it sound so trivial. "Oh you're just seeing it as an accessory, to make your relationship considered legitimate." Uhhh no, that's not trivial that's a pretty big deal.
"Consider 18 year old boys (or girls) getting gay married, at what point does it get a bit ridiculous?" - Why should I consider that? Is it a major problem or something that I'm unaware of? Is there a rush of 18 year old boys getting married to each other? Is it in some way ridiculous? Or did you just completely make that up? Is that the part that is a bit ridiculous?
"The question is: what IS marriage?" - Uhhh no... the question is, who the fuck are you to decide that my relationship doesn't mean as much as yours does?