Example: Now that Ellen DeGeneres has married Portia de Rossi, would I write to "Mrs. and Mrs. Ellen DeGeneres"? "The Ellen DeGeneres Family"? or "Ellen and Portia DeGeneres"?
Abby's answer -- ask them how they want to be addressed! -- is correct as far as it goes, but neglects to challenge the presumption of "Straight But Not Narrow in Glendale" (and the person who wrote the column's headline) that this problem of address is new and unique to same-sex couples. But it applies to opposite-sex couples too. For example, my wife and I both kept our original names. Therefore, it is incorrect to address cards and letters to "Stentor and Christina Danielson," and extremely incorrect to address them to "Mr. and Mrs. Stentor Danielson." (When we get letters like that, we joke that the writer has inadvertantly revealed the existence of my secret second wife who did change her last name.) "The Stentor Danielson Family" would be troublesome as well (since it still implies that I am the singular head of the household), but I would accept "Stentor Danielson and family" if the writer is primarily interested in me and/or is unsure of the number and identity of the other people in my household.
Same-sex marriages make unavoidable the fact that marriage can be (or at least, can be intended by the members to be) a partnership of equals, not an institution with two distinct-but-complementary, gender-assigned roles. Though opposite-sex couples can try to put their relationship in that egalitarian mold, it's still easy for outsiders to treat them as if they fit the gender-differentiated pattern -- e.g. it's clear that if we were to be a gender-differentiated couple on the name issue, then "Danielson" would be the last name we'd share. Some people find that total loss of formal gender role differentiation morally objectionable, and launch attacks on the "redefinition" of marriage by gya rights activists. Others simply take certain elements of gender-role-differentiated marriage so for granted that encountering egalitarian marriages just confuses them -- as seems to be the case for "Straight But Not Narrow in Glendale" (and for my grandmother, who once apologized for addressing a Christmas card to "Mr. and Mrs. Danielson," since she knew my wife kept her last name, but said she just didn't know how else to address it).