Just don't call it marriage.
By the way, if memory serves, the leadership of the gay rights groups weren't that interested in gay marriage as an issue, five or ten years ago. They wanted to work on civil rights for gay people for employment, which they figured was do-able politically. Plus, some of the leadership of the gay groups are into the notion that marriage is contrary to the idea of sexual liberation. The gay marriage issue suddenly appeared a few years ago because individual gay couples decided to sue certain jurisdictions for the right. The leadership of the Democratic Party were not happy campers, correctly foreseeing that this would cost them close elections. Here in the Louisville area, the big local megachurch worked hard to get out the anti-gay-marriage vote, and this may have turned a very close senatorial race to Republican in Kentucky.
I agree that the two specific rights you mentioned should be allowed, but once any right is allowed it will be easier for them to get more. The floodgates will be opened as it were. I think a person can have anyone be their power of attorney already anyway. I don't care if they call it marriage either, it's just a word, they could call it anything for all I care. It's just the "rights" I think that will explode the issue.