Any relationship you have might be good enough for civil unions, but not for marriage.
At Box Turtle Bulletin, Rob Tisinai talks about why having marriage, not just civil unions, is important:
I have to think segregation of straight and gay relationships has a detrimental effect on gays. It denotes the inferiority of gay relationships. It leaves gays with an attitude of futility when it comes to commitment.
Compare also the Supreme Court brief by Olson and Boies.
This year Ireland, as part of its new civil partnerships law, decided to recognize marriages – and similar institutions – from other nations as civil partnerships within its borders.
Ireland didn’t miss any country which recognises same-sex marriage, but it did miss various other “civil union” and “domestic partnership” arrangements, even some which were identical to marriage in all but name. Terminology: it matters.