Sounds kinda weird to describe it that way. If you are seriously dating with the prospect of marriage or actually married; both of your assets are joined. All money goes into one pot, so there isn't a 50/50 dynamic. If one person is paying utilities, the other is funding the retirement plan. If one person is paying rent/mortgage the other one is paying daycare. It's one big pot.
Now if you are just casually dating (like most people), then you shouldn't be sharing bills at all. Shouldn't even be living together. She won't go 50/50 with you cause she should be paying 100% of her bills. She is not a child.