What is the philosophical/theological logic for why unconsummated (ratum tantum) sacramental marriages can be dissolved?
Spiritual and physical bonds of marriage
before consummation there is only a spiritual bond between the spouses, but after it there is a physical bond between them as well. And therefore just as after consummation marriage is dissolved by physical death, so also it is dissolved before consummation by entrance into religious life: for the religious life is a certain spiritual death, by which someone, dying to the world, lives for God.
Spiritual and physical significations of marriage
St. Thomas gives the traditional significations of marriage,
- unconsummated (spiritual) marriage → union of Christ to soul by grace (can be broken by sin)
- consummated (physical) marriage → hypostatic union of fleshly human nature with the Second Person of the Holy Trinity (absolutely indissoluble),
Before consummation marriage signifies the union of Christ to the soul by grace; which indeed is dissolved by a contrary spiritual disposition, namely, sin. But by physical intimacy is signified his union with the Church by the assumption of human nature into the unity of his person, which is indivisible in every way.
The state of celibacy (being unmarried) is higher than the state of marriage, so the Church allows those in even consummated marriages to separate in order to enter religious life and advance in charity.
Whether matrimony can be dissolved by entrance into religious life?
Before consummation one’s body has not been transferred to the power of the other absolutely, but under the condition that in the interim the other spouse does not aspire to the fruits of a better life. But by consummation the aforementioned transfer is completed, for then each person enters into bodily possession of the power handed over to each other. This is also why before consummation one is not bound to render the debt immediately after the marriage is contracted in terms of the present, but the time of two months is given to them, for three reasons. First, so that in the meantime he might deliberate about entering religious life. Second, so that the necessary things may be prepared for solemnization of the wedding. Third, lest the husband should hold cheap a wife whom he did not have to pine in wait for.
quoted in Ford, S.J., Validity of Virginal Marriage pp. 110-11
Marriage is a contract
by which each party gives and accepts perpetual and exclusive rights to the body, for those actions that are of themselves suitable for the generation of children.
1917 can. 1081 §2; cf. 1983 can. 1057
If those rights/privileges are used/exchanged by the partners, the marriage cannot be dissolved:
However, if they are not, then
A non-consummated marriage [matrimonium ratum tantum] between the baptized or [a marriage] between a baptized party and a non-baptized party can be dissolved by law upon solemn religious profession, or by dispensation granted by the Apostolic See for a just cause if both parties or [just] one ask for it, even if the other is unwilling.
1917 can. 1119; cf. 1983 can. 1142
Commenting on 1983 can. 1142, Juan Fornés writes (Exegetical Commentary on the  Code of Canon Law vol. III/2 pp. 1546-7):
5) With respect to the basis of the absolute indissolubility of a ratified and consummated marriage, in contrast to the possibility of dissolution in other marriages, the specialists have not yet offered an entirely satisfactory explanation. This is logical, if one bears in mind that this is a subject in which the sacramental nature of marriage comes into play and in which, therefore, there are facts of faith that exceed purely speculative human limits. We are in the very field of mystery, in which the magisterial teachings are decisive.
Nevertheless, from a juridical point of view, some clues can be provided. The first highlights the importance of consummation in sacramental marriage by considering consummation as a juridical category ("a juridical fact, which strengthens the indissolubility of the bond"9) that affects sacramental marriage, to the extent that it contributes to the sacramental symbol. Hervada stresses: "It is not a transaction consummation but rather a sacramental consummation. The characteristic firmness that, due to the sacramental nature, is brought about by the first conjugal act is not in the order of the consummation of juridical transactions, but in a singular order of efficacy that cannot be reduced to the usual categories of the effects of the consummation of real or consensual juridical transactions."10
The second considers indissolubility in light of the essence of marriage as one flesh: "the significance of a consummated marriage does not lie anywhere but in the very fact that the spouses have ontologically expressed themselves as one flesh, a reality similar to the union between Christ and the Church through Incarnation."11 It also considers the inseparability between marriage and sacrament: "The fullness of the significance creates a correctness of the juridical value of the bond, attributing to it a firmness that it does not have only because of the pact, giving it an indissolubility that makes it similar to the indestructible union of Christ with the Church."12
- J. HERVADA, "El matrimonio canónico. Teoria general," in Derecho canónico (Pamplona 1975), p. 394.
- J. HERVADA-P. LOMBARDÍA, El Derecho del Pueblo de Dias, 111/1, Derecho matrimonial (Pamplona 1973), p. 303.
- Ibid. , p. 303.
- Ibid. , p. 304.