Haha, a few reasons.
1) I'm twenty-three. Give me a few more years.
2) He totally just got out of a really messy divorce a few years ago, and I doubt he REALLY wants to be married again.
3) The gay marriage thing in Canada is still a little bit sketchy. I don't want to get involved until I'm sure it's not going to crumble in a few years.
and 4) If he's going to ask me, he better do it a little better than casually asking, "Do you wanna get married?" when we're in our pyjamas and he's covered in bicycle grease. Puh-leeze.
EDIT: Oh, and I forgot. 5) We'd argue endlessly about last names. I happen to like my patronymic, it's part of my heirtage. In Norway (at least in my family), when you get married, you keep your last name, since it's your father's name. But Matt would probably want me to take his, and I would argue.