The Vatican document explicitly saying Catholic priests can bless same-sex unions lays out the conditions for what such blessings can, and cannot, involve.
The overall goal is to make it abundantly clear to the couple and those around them that the blessing is not a liturgical or sacramental ritual, and that it in no way resembles a marriage. This is because the Catholic Church teaches that marriage is a lifelong sacramental union between a man and woman.
