First of all, even if Elton John's statement that "You get the same equal rights that we do when we have a civil partnership" were true in the legal sense (which it isn't), there's more to marriage than the formal legal rights involved. Marriage is also about informal social rights: the right to introduce your spouse with a label that everyone understands the exact meaning of, such as "husband" or "wife," rather than a label like "domestic partner" that nobody quite knows the laws regarding, nobody quite knows the seriousness of, and nobody anywhere can imagine whispering it into a beloved's ear - "My beloved domestic partner! I love you so much! You're the most beautiful domestic partner in the world!"
But a second, less understood problem with the "separate but equal" approach is that when two groups' legal rights are kept "separate but equal," the formal legal rights invariably end up unequal as well. For example, elfwreck recently posted in the gay_marriage community a list of nine differences in the legal status of California domestic partnerships versus marriages. These are nine differences under California state law, in addition to the many, many more differences resulting from the fact that federal law doesn't recognize either domestic partnerships or same-sex marriages. (And when federal law eventually does get around to recognizing all marriages equally, as President Elect Obama said last summer that he would support, it almost certainly still won't recognize any "separate but equal" partnership types created to segregate queers.) And these differences in California state law's treatment of domestic partnerships and marriages persist even in spite of the fact that California state law also specifically states that "Registered domestic partners shall have the same rights, protections, and benefits, and shall be subject to the same responsibilities, obligations, and duties under law, whether they derive from statutes, administrative regulations, court rules, government policies, common law, or any other provisions or sources of law, as are granted to and imposed upon spouses."
To name just a few of the most egregious of the nine differences: Domestic partners in California are required to live together before they can register as domestic partners. Can you imagine the outcry if heterosexuals were required to live together before they were allowed to get married? This requirement feels to me like a deliberate effort to run the "living in sin" notion in our faces. Also, marriages are legally solemnized in a ceremony with witnesses, signifying the involvement of society in celebrating and supporting the marriage; domestic partnerships are registered just by filling out some paperwork, signifying that theirs is a contract between two individuals alone, that the rest of society does not celebrate. Couples can have a "confidential marriage" in California, in which no record of the marriage is ever made available to the general public, but domestic partnerships are required to be public - the benefits of domestic partnership come only at the cost of making oneself a public target for potential discrimination.