Making it official

you can't certify love so it would last longer. You can make it "official" all you want, in long term it won't make you love someone you start to hate.

I don't see why you'd see it as a bad thing.

I don't see why it's any good either, all it does is complicates everything if you ever decide to leave. Standing in front of people and saying that you won't leave is an empty promise that is easy to make when you don't have to make good on it for the rest of your life at that exact moment.

Your relationship is not going to change because of marriage, if you would have stayed with that person without being able to marry them same thing would happen. If you would have realized you don't want to be with them that is also not going to change. So all you are left with is spectacle and empty promises. You have no idea what is going to happen, you can stand there and make the most romantic promises in the most beautiful clothes in from of everyone you have ever known but it won't change anything if your wife cheats on you or your husband treats you like a piece of shit.

Tax benefits?

yes, government would really like to tie you two together promising perks and hoping you will create a new workforce. This is the real reason to get married, government wanting some babies and being willing to pay for it.