This is an interesting question.
My fiance and I have been sharing expenses for about the last three years of our relationship.
When we were in college, we both worked, and had extra income because we weren't paying expenses like rent and everything like that. So we took turns paying for each other because we could.
Then he went his final year of grad school without a job. I added him to my cell phone plan and paid his cell phone and he paid for everything else with his savings.
Then we were both working again. And were back to paying for our own lifestyles and such. Every once in a while we would pay for extra things for each other and what not.
I got into a car accident and was out of work. He took over ALL of my finances at this point so I could finish schools since I was unable to work at this point anymore. He asked me what I needed in a month. I was literally getting a monthly stipend from my fiance to pay things like my car payment and insurance, our cell phone bill and groceries!
All of this happened BEFORE we were living together.
Now, we are living together and he makes A LOT more than I do. His career is set and I am still working on getting mine rolling. I work a few hours a week and have been doing great internships, that don't pay or pay very little.
The way we have worked this out is he pays all the bills. I take whatever paychecks I receive and put them into our joint savings account. We have decided on a monthly allotment for food and other budgeting needs while we are saving for the wedding. I take care of cleaning the house and cooking when I am not doing my internship because I have more hours at home. He does things on the weekends around the house because that's when I work.
When we moved in together, we combined our finances. Yes, we both have our own credit cards and bank accounts, but we also have join checking and savings account. Our money at this point isn't his and hers, it is ours. We put it where it needs to go.
I don't spend anything extra or frivolous without consulting him first, and he does the same even though he makes the majority of the money in our household. I will be making more money that him over the summer time because of the career path. We won't change how we pay for things though. My money will still go into our savings account and his will pay for the bills. This way we have that extra money for whatever we want to do.
It's not fair to require one to pay an equal amount if it leaves you struggling. But if you can't pay, you should do something else. Clean the house, do the shopping, cook the majority of the meals.
Even if the balance isn't in cash, it still needs to be there in other forms of your relationship. I would agree with the other posters about taking a premarital course before marriage. They help you understand finances as a couple. It is one of the leading causes of divorce in the US. I think it would benefit both of you and then you can understand each others opinions about how to handle finances and what you can change to make your lifestyle better for you as a family unit versus individuals.
Good luck!