Even in the case of marriage, why is it expected to keep the money even?If one makes more than the other, is it expected then that the one making less pitches in in a different way to make up for the fact that they're making less?It seems at one point in time the even split in assets made sense, since there were pretty well defined gender roles. But, if both are working, and doing house work, and cooking, and taking care of kids, why would the money just be split 50/50? Everything is split equally, except the money making, why doesn't the one making more get to keep that extra share?
I am not married, and entirely willing to hear from the married folk that I don't know what I'm talking about. I trust you will let me know if you disagree.
Why keep the money even? Because having extra money on one side of the partnership is the same as having extra power on one side of the partnership. One person's reserving extra power to herself makes both people feel like they aren't in this together. When you keep a little power to yourself, you are keeping the ability to get your way just a little more often and both people will notice that. Because most people's conception of marriage is that they are both working together against the rest of the world and they pool their resources to do that. Keeping extra money to yourself is also reserving just a little advantage as against your partner, suggesting that the marriage isn't the two of you against the world, but one spouse against everyone, including the partner. Because a marriage is a gift of yourself to your beloved, and that includes the money you earn.
All that said, I personally like a different allocation rule. While I can't guess how it will actually work out when I am faced with this in real life, I like the idea of both partners putting two-thirds of each paycheck in a joint account and reserving one-third to themselves. 'Cause why argue, or even discuss, some personal purchases? Buy it for yourself with your own money, and look, no one cares! It would also make gifts more meaningful, if it came from your own hoard rather than shared money. And give you a reserve, if you are the cautious type.
Also, is it creepy if I wanted to give my imaginary fiancé a gift of, say, an hour's time with a divorce attorney before the marriage? I bet you think it is. But I took a semester's worth of Marital Property. I know how property is allocated between spouses, during the marriage and after dissolution. Some of that is pretty surprising. It doesn't seem fair for me to know all that, while a non-lawyer imaginary fiancé doesn't. (See? Not preserving a power advantage.) Wouldn't it be an unromantic kindness to get that evened out before a wedding? Or too creepy?