Secrecy can be a killer. Many couples get married in their late 20s or even early 30s and are used to being independent. Having separate accounts allows the couple to maintain some financial independence while being completely open about it.
There are about 1,100 laws that apply only to married couples that more than make up for the marriage penalty.
Newlyweds make decisions that affect each other. Young couples need to think about the unthinkable. If you have assets that you want your spouse to keep if you die, you need insurance.
Many young couples are so busy being romantic that they forget to talk about anything practical like personal finance. Money isn't a romantic subject, but marriage should be seen as entering into a financial as well as a romantic partnership.