In a position statement (probably from a few years ago) on the nature of marriage and its recognition and regulation by civil society (see "One Man, One Woman, For Life: Lead Messages on Marriage Redefinition"), the U.S. Conference of Catholic Bishops stated the following about "civil unions", etc.
Civil unions, domestic partnerships, and the like are non-marital arrangements that mimic aspects of marriage. They erode the institution of marriage and overlook the essential contribution that marriage makes to the common good. They cannot be approved or promoted.
This perspective, perhaps taken out of the context in which it was originally formulated, goes far to explain any opposition that the Church in the U.S. may have determined against such purported solutions prior to the Supreme Court's Obergefell v. Hodges case. By an interesting twist, progressive activists preferred to avoid such solutions, as well, preferring to lay claim to the term "marriage" itself. In view of the new legal climate, however, it strikes me that believing Christians need to ponder what options remain to them.
The Church gives a succinct definition of marriage in its Code of Canon Law.
Can. 1055 § 1: The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life and which is ordered by its nature to the good of the spouses and the procreation and education of offspring, has been raised by Christ the Lord to the dignity of a sacrament between the baptized.
Can 1056: The essential properties of marriage are unity and indissolubility, which in Christian marriage obtain a special firmness by reason of the sacrament.
Notice that Canon Law does not declare fecundity to be an essential property of a marriage. Couples who are unable to conceive are not, therefore, deprived of the remaining blessings, and obligations of marriage.
The fact that marriage is inherently limited to heterosexual couples and "ordered by nature to the good of the spouses and the procreation and education of offspring" is not something subject to the whim of governments. it is what it is.
Obviously, that doesn't prevent government from stating something incompatible with the above view in its laws. The U.S. Supreme Court, by a 5-4 majority in Obergefell v. Hodges, did precisely that. In effect, the Court decided to change the definition, the properties and the purposes of marriage.
In particular, the term "marriage" as determined by government no longer has the same meaning as the term "marriage" as known to the Church. I have already noted in a prior posting that this makes it illicit for a priest or a deacon of the Church to continue in the role of minister of the government in the matter of marriage. It also ensures that a baptized couple who seek to be married by the state delegitimize their intention to marry in the sight of God by, in effect, claiming that the two definitions are compatible.
Because of this, and because a couple who wish to marry still have a legitimate need for recognition by the government of natural privileges and responsibilities of marriage, it would seem appropriate to revisit the Church's stance against "civil unions" and the like. Indeed, the reasons originally put forth for the Church's opposition no longer fit the current context.
In the current post-Obergefell v. Hodges context, it would help to advance the practical protection of marriage if the civil authority enacted any category of contractual connection such as the following.
License to cohabit (referred to colloquially as a civil union or partnership): Any partnership of two or more individuals who share a life together, intending (for the most part) to share the same residence, share living arrangements and expenses, etc. who shall, by default, grant privileges and responsibilities such as inheritance, debt obligations, hospital admitting privileges, etc., unless specifically determined otherwise by established law or contract. In the case of such a practical living arrangement, the government could choose to permit joint filing of tax forms.
License to procreate: Any partnership involving two or more individuals who are pairwise capable of acts of procreation agree to accept the primary privileges and obligations of parenthood, should such procreation be successful. Such obligations include, but are not necessarily limited to, emotional, physical and financial support, as well as academic and moral formation. In the case of the dissolution of said license, these responsibilities toward children remain (and parental visitation rights, an essential privilege of carrying out such responsibilities, remain) until such time as it may be agreed that the children in question are adopted by one or more substitutes. Government may decide whether a given arrangement is to be considered exclusive and unchangeable, or whether it may be amended as requested by the parties concerned. Such contracts would have, as their primary objective, the protection of the natural rights of children. Such protection, in its fullest extent, would, of course, depend on those undertaking such a contractual arrangement to do so only in the case of they're being a heterosexual couple who intend their arrangement to be life-long and exclusive.
It should be understood that these two categories of contract are mutually independent, not mutually exclusive. Any parties may choose, by mutual agreement, to enter into either, both or neither. In both cases, government could choose to accept either the dissolution of such contracts or their temporary suspension or their amendment, as agreed upon by all parties thereof or as determined by a court as necessary for the resolution of disputes. Morality need not play a role in any of this, if that is what government should decide. The primary purpose for a morally inclined heterosexual couple to enter into either contract would be to protect their natural rights from the encroachments or limitations of government or private entities. There would be no necessary legal connection between their marriage and their contract, thus preserving the integrity of the former to the extent that the civil contract(s), in the detailed provisions, is(are) compatible with it. At the same time, the government would avoid any obligation to protect or even to recognize the marriage, as such. Such contracts, entered into in one state, could automatically be recognized (by the principles established in legal interpretation of the commerce clause of the Constitution and the "equal justice" provisions of the 14th Amendment) by other states governed by the same Constitution.
Married couples would not be morally obliged to enter into any government sanctioned contractual agreement, but would do so solely for their convenience and/or protection. No signature or collaboration from a religious minister would be required or needed to establish or formalize such a contract, thus, in particular, implying no religious sanction.
Under such a schema, government would give no particular sanction to marriage, as such, but would avoid, from a legal perspective, harming the marriages of morally conscientious heterosexual couples. Government, furthermore, would remain aloof from the moral legitimacy of any given arrangement, leaving that solely to the judgment and responsibility of the contracting parties.
In order for a "license to procreate" to be a benefit to society, government must not prevent a heterosexual contract of this nature if both parties are law abiding citizens of suitable age and freedom to undertake such a contract. One can, of course, envision a time in the future where a "progressive" government may choose to limit access to such arrangements to a "suitable number" or a "suitable genetic pairing", but such government imposed limits would be opposed to the natural rights of its citizens. Similar, though not identical, considerations apply to the case of government not limiting access to a "license to cohabit."
To the reader, I submit that the above proposals would make it possible for government and religion to coexist peacefully and for the natural rights of its citizens to be protected in spite of the ambitions of so-called progressive change-agents. It would not, and cannot, guarantee such protections, since it also enables non-heterosexual couples to enter into such contracts and, perhaps may legitimize, by default the adoption of children into non-heterosexual contractual regimes. Participation in such a legal framework would necessarily imply what is called "remote material cooperation with evil" on the part of citizens who chose to live under such a regime. Formal cooperation with evil, however, would be more limited and more directly under the control of those who chose to collaborate directly with immoral contractual arrangements.
Unfortunately, this is an inevitable and unavoidable consequence of government choosing to protect "positive rights" rather than "natural rights." Indeed, in the above analysis, I have largely avoided consideration of the moral obligations of citizens. For a specifically moral perspective, I can recommend the reflections of Archbishop Joseph Kurtz, President of the USCCB, in "Where do we go from here?"