The following interesting analysis is by Francis Beckwith:
Up until very recently, marriage had been universally thought of as consisting of three essential characteristics: conjugality, permanence, and exclusivity. This had been more or less reflected in our laws.
Conjugality refers to the way by which a marriage is consummated: coitus between the male and female spouses. Permanence means that the marriage cannot be dissolved (or annulled) unless certain specific conditions are met or one of the partners dies. Exclusivity refers to the sexual relationship and means that neither party in the marriage is free to engage in extra-marital intimacies.
Even polygamous unions may fulfill these criteria, for the husband is married to each wife while the women are not married to each other and thus do not have “wives.” For this reason, upon the husband’s death, each of the marriages in a polygamous cluster is immediately dissolved.
Conjugality is only a condition because of the nature of sexual intercourse: it is ordered toward bringing into existence offspring of the union of the two parties. This is why handshakes, hugs, kisses, or other forms of bodily touching, penetration, or intimacy can never count as conjugality.
But this is also why it is wrong to say that the latter are indistinguishable from conjugal acts that cannot bring forth offspring due to illness or age. For such conjugal acts, though sterile, do not cease to be conjugal acts, just as a man in a coma does not cease to be a rational animal simply because he is not able to exercise his rational faculties.
Just as the comatose man still possesses human dignity even though he is not able to exercise his unique human powers, the conjugal act between a husband and wife that is incapable of conceiving possesses no less dignity precisely because it actualizes the same profound and mysterious union that is by nature ordered toward bringing into existence a unique and irreplaceable person that literally embodies that union.
Given the sacred nature of conjugality, as it was once without controversy believed by virtually everybody, exclusivity and permanence make perfect sense, especially if one also believes that children are not only best raised, but by nature entitled to, a mother and a father who bind themselves to each other under the authority of a covenant whose contours they are not competent by mere consent to dissolve or change.
Of course, all of this – conjugality, permanence, and exclusivity – is not only no longer taken for granted, it seems literally incomprehensible to most citizens morally formed in a post-1960s culture.
The Pharisees Question Jesus by James J. Tissot, c. 1890
With the enactment of no-fault divorce laws beginning in the early 1970s, permanence began to dissipate. Exclusivity quickly followed. The sexual revolution brought with it not only the unraveling of the mores against fornication, but also the concepts of swinging, open “marriage,” and even polyamory. So, violations of exclusivity slowly became understood as not intrinsically wrong, but rather, only wrong insofar as the spouses do not grant their mutual consent.
Thus, not unexpectedly, came the proliferation of out of wedlock births, single parenthood (with children from a variety of male and female suitors), broken families, and blended families.
Consequently, nothing special in marriage remained. Every relationship in the familial tapestry can in principle be severed, reattached, or removed as long as all the adult agents consent. However, those without capacity (i.e., children) may be eliminated (by abortion, if not post-natal) or, along with any common property, “equitably” distributed to interested parties by the state in judicial proceedings firmly committed to enforcing the couple-relative contours of this new institution that is called “marriage.”
Given this trajectory, why should conjugality remain? That is precisely what is implied in the public rhetoric of supporters of the legal recognition of same-sex unions. It makes perfect sense to many of our fellow citizens, in light of their personal experience of having grown up in a culture in which they were taught that “marriage” is virtually an artifact shaped by our disembodied wills rather than a sacred institution that we did not invent and under which our embodied wills are bound when we enter it.
But this brings us to one of the great ironies of our age. Recently, a baker in Colorado was told by the state that he may not refuse to make and decorate a wedding cake for a same-sex couple who had been “married” in Massachusetts, but had planned a reception in Colorado, which presently does not legally recognize such unions. The baker had refused because he could not in good conscience offer direct material support for a liturgical event that his theological beliefs affirm as gravely immoral.
Oddly, with the demise of the conditions of exclusivity and permanence, and now conjugality, this ruling means that each partner in a legally recognized marriage (or civil union) literally has less of a legal obligation to each other than the baker has to the couple. Apparently, the state believes that preserving the relationship between baker and same-sex couple is of much greater importance for the cause of public justice than doing the same for the relationships it claims to be vindicating.
Or to put it another way: It is more difficult in Colorado for a baker to leave some customers than it is for the partners of a “married” couple in Massachusetts to leave each other.