Pros:
Umm, you have a 'life' partner.
Umm, you get to have a nice wedding.
Umm, you get to wear a shiny ring.
Umm, it shows that you're 'committed' to your relationship and you love someone enough to want to spend the rest of you life with them.
Umm, you will no longer be 'shacking up'.
Umm, it might look better on paper.
Kids
Cons:
If you decide to break up, the divorce process can be messy.
Weddings are expensive.
Gotta pretty much run everything you want to do by your spouse. But I guess if you don't mind that, it won't be a big deal.
Everything gets split down the middle whether you worked for it or not.
Gotta take your spouse everywhere.
Lying becomes harder.
Kids.