Equal rights for whom?

The same-sex marriage debate increasingly calls, or should call, into question the legitimacy of the privileges bestowed on marital relationships in general. Take this entry in Andrew Sullivan's blog (scroll down to "Why civil unions suck"). Sullivan quotes a letter from an army wife saying that when she and her now-husband were still dating and he was deployed to South Korea, she found that, despite having legally notarized power of attorney, she was repeatedly thwarted while trying to handle his affairs. She also found out that, without being legally married, she would not even be notified if he was wounded.

This doesn't quite make Sullivan's case for full marriage vs civil unions—many of the current proposals for civil unions specify that the couple would have the same rights as a husband and wife. But apart from that, what is the moral and legal justification for the exclusive rights of the marital relationship? Does this mean that a single soldier deployed overseas will find it much more difficult to keep his financial and legal affairs in order than a married soldier, because whoever he entrusts with those rights will not be able to carry them out as effectively as a spouse?

Or take the issue of estate taxes. Right now, a wealthy 80-year-old widow can leave all of her property tax-free to a 20-year-old boy toy if she marries him a month before her death. If the same widow wills her property to, say, her niece who has been living with her and providing constant care and companionship for 10 years, the niece will have to pay a huge estate tax. Where is the justice in that?