The only problem I can see is that if you're (presumably by the age of your cat) 18 and have been with the same person since you were 14, you probably haven't tested the waters to see what's out there all that much. I think it's hard to know if you really love someone until you've been with a lot of people you definitely do not love.

However, no one can predict the future to know how happy your marriage will be, so congratulations. I'm not sure about the cat as best man though. It seems to me something that will be funny in the moment and for a year or so, but when you watch the wedding video in a decade will seem a bit kitchy.