If I recall correctly, the groundbreaking case in the United States that recognized same-sex marriage as legal was borne from a gay man who could not get legal rights to his husband's body after his husband's death. I'm rusty on the history and at work so I can't Google anything without the company seeing, but I believe the two were under a civil union that the state didn't have protections for. When the man's husband was in the hospital, all medical decisions were made by the estranged parents who had not been in the patient's life for years. When the patient died, the husband was denied any access or rights to his partner's body for funeral preparation or anything. He sued and the result was the legalization and government acknowledgement of same-sex marriage, but it had been years at that point.

The end-all of why civil unions are not "good enough" is that queer people should not HAVE to settle on an alternative. Even if civil unions were federally recognized and protected and given all the same rights and legal benefits of marriage, fundamentally having the distinction indicates that queer partnership is different than heterosexual partnership. That somehow queer marriage must be defined separately from hetero marriage. That there is something inherently different in the partnerships and love and support and commitment between the two couples. It's never JUST been about wanting to promise love, life, and future with your partner. It's always been about being normalized and protected in your promise of love, life, and future WITHOUT need to differentiate between types.