There is no rational reason other than business to get married. Even the business aspect is a risky venture at best, who knows what financial pitfalls and hardships lay ahead.
So be bluntly honest
1. We get tax incentives, stability, standard of living, inheritance, etc.
2. Since we do not have any genuine new ideas, why not do the same everyone else does? What else should we do? We both love and are comitted to each other, now what?
3. You get to plan the wedding.
4. We may get lots of cool wedding gifts.
5. I promisse that on the day we get married, after opening all the gifts, I will let you know is there is a value in public rituals to express and deepen our bond with each other.
6. In other words, there aren't any genuine reasons for it. But staying single? It costs you. It really does. If you stay out of this government/religious promotion of a lifestyle, it is very hard to get by, without its perks.
And it sounds better than civil union.
7. We are both held in legal responsibility for one another - should you do something terribly crappy to me, I have some legal ground to sue for damages. Now! I ask you, isnt that nice and considerate from the gov?
You can say all that, or....
I value a stable family and a lifelong committment because it leads to a better environment for our children and a happier and less stresful and, hopefully, more fulfilling life for ourselves. Marriage just so happens to be a possible answer for this. Are you in? Or, are you out?