One of the requests I received when I asked for them a couple months ago was for an account of Thomism on "the exclusivity of love (IOW, why should it be limited to one person only)." I have been puzzling about how to handle this request, but I do think I should say a few things about it.
(1) There is no general exclusivity of love in Thomism; we are to love God and our neighbor, and because the latter love depends for its fullness on the former, our neighborhood is all creation. Now, the request was specifically about sexual love, but Aquinas himself doesn't recognize any specifically sexual love. There is love as an act of will (the kind of love we are commanded to have) and love as a passion, and sexual love is at least the latter in a sexual context, and hopefully the former as well. Love as such has no direct implications in the matter. Love is just complacentia, the being pleased with, an object, that results in being drawn to the object (which we call desire) and joy in attaining it.
(2) There is, to be sure, a distinction in love-as-a-passion between love of concupiscence and love of friendship, and this does give a relevant consideration. In terms of effect, we could summarize the difference in more modern terms by saying that love of concupiscence, craving love, results in jealousy and love of friendship results in zeal for another's good. The former of these clearly does have to do with exclusivity: if you are jealous, you want some good to be exclusively yours somehow. This does come up when Aquinas talks about exclusivity in marriage, as a sed contra, but it is not the mere fact that it happens but that we universally take at least some such jealousy to be a natural consequence of the kind of sexual love in a marriage. That is, it is not jealousy as such that does the work, either. It's the structure and character of marriage itself that actually makes the jealousy natural.
(3) We have to be very careful in talking about Aquinas on marriage, much more careful than people usually are. We do not actually have Aquinas's final account of marriage. The Summa Theologiae cuts off before it gets to marriage; the question in the Supplement is, like all the questions in the Supplement, excerpted from Aquinas's Sentences commentary, which was his earliest work. This is actually something of a serious problem with much of the Supplement, since the questions on the sacraments that we have show considerable and important revision over the corresponding questions in the Sentences commentary. We have quite literally no idea how St. Thomas's Summa-era ideas on the instrumental causality of the sacraments would have forced the revision of arguments in the question. It would have had to, although much would have been retained; but since Aquinas never actually got around to revising them, we don't really know how it would have played out. There are scattered comments on marriage in the Summa as such, whence we can confirm some of the principles that would have been presupposed: matrimony is both an office of nature and a sacrament, it contributes to the completion of the whole community of the Church both naturally and spiritually, it is concerned with procreation and with remedy of concupiscence, as a sacrament it gives grace by signifying the Passion of Christ in respect of its compassion, it is appropriately treated as the last of the sacraments because it is the least spiritual sacrament, but it nonetheless super-excels all the other sacraments on the specific point of signification. None of this is particularly surprising and, more to the point, none of it addresses the exclusivity of marriage.
In the Sentences commentary and the Supplement, marriage is exclusive in a very complicated way. St. Thomas thinks that having many husbands and having many wives are not perfectly symmetrical when it comes to marriage as an office of nature. Marriage as an office of nature has two ends, of which one is the begetting and rearing of children and the other is community of life. Having many wives does not hinder the first end of marriage at all. Nor does it necessarily hinder the second, although it is very difficult to arrange things peacefully enough so that it doesn't. Thus on Aquinas's view as presented in the Supplement, polygamy, taken to mean a husband having many wives, is not rigorously contrary to natural law. It's just that permissible cases are necessarily rare. This is not the case with a wife having many husbands, according to Aquinas, because such a situation has the additional feature of complicating the connection between fathers and offspring in such a way as to interfere with the raising of children. (He also thinks that it increases the chances of the woman or the fetus being harmed in pregnancy.) It is only in marriage as a sacrament that we get a clear and clean rejection of polygamy in both senses: matrimony cannot function properly as a sign of Christ united with his Church if it is polygamous, nor is polygamy well-suited to raising children in the faith. This is a fairly layered discussion, which approaches the topic without any of the more sophisticated nuances of Aquinas's later sacramental theology. It is also remarkable in that Aquinas explicitly affirms that some kinds of polygamy are on rare occasions perfectly consistent with marriage as an office of nature, although none are consistent with it as a sacrament.
In other words: in Aquinas's account as we have it, the exclusivity of marriage is, strictly speaking, purely a matter of the relation of Christ to the Church. It's highly conformable and appropriate to natural law, as witnessed by the occurrence of mutual jealousy and by common customs; but it is not strictly required by it.
When we look a little more widely at Aquinas's discussions of sexual sins, we find little more help. Again and again the major issue in (say) fornication or adultery is not the nature of love, nor even the nature of marriage, but injustice to any children that might result from the sexual activity. That is to say, to commit adultery or to fornicate is primarily wrong because it is to act in such a way that if any children result you have already committed an injustice against them. Occasionally there are suggestions that injustice to the mother, should she become a mother, is also a factor somewhere in there. Exclusivity of love doesn't really enter into the discussions, at least any of them I can think of.
Thus there is no general exclusivity of sexual love in Aquinas; and marriage is only intrinsically exclusive for Aquinas when it is sacramental, although he holds that there are almost always good reasons for rejecting polygamy. (Sexual relationships other than marriage, like concubinage, are of course always contrary to natural law. But here again, we see that it's really marriage that's important to the question, and nothing else.)
(4) But this is all focusing on Aquinas himself, and again, St. Thomas's failure to finish the Summa leaves us with an account of marriage based only on his very earliest ideas on the subject, along with some later scattered comments in the Summa and elsewhere. This is not much to work with. However, Thomism is broader than Aquinas. I haven't done any thorough investigation, but the traditional commentators almost certainly just stick with Aquinas. Cajetan, for instance, while opposing Henry VIII's divorce, pointed out that polygamy could possibly be an acceptable solution to the problem.
I think to find a form of Thomism that has a much stronger view on the exclusivity of marriage, you'd probably have to look at something like Lublin Thomism, the sort of thing we get in Karl Wojtyla's Love and Responsibility, although L&R itself only definitely rejects polygamy on the grounds of the Gospel, and seems to me to be mostly vague otherwise. But I haven't looked very thoroughly at possibilities there, either.
(5) So I suppose this all boils down to the response that if you want something on the exclusivity of sexual love, or even the exclusivity of marriage, Thomism is not at all the place to look, because Thomism in general has no strong position on the subject, and actually inclines against the strict necessity of any such exclusivity, although affirming that it's usually far and away the better idea. This is certainly not what most people would expect of Aquinas and Thomism a priori, but there it is. Thinking that polygamy is on rare occasions perfectly fine is entirely consistent with even the strictest Thomism. There is no general Thomistic account of the exclusivity of love, although that doesn't strictly rule out the existence of some particular form of Thomism having such an account.
Incidentally, it's worth comparing Aquinas on this subject to Scotus. Both Aquinas and Scotus agree that on rare occasions polygamy may have dispensation. However, dispensation on Aquinas's account is simply the fact that secondary principles of natural law don't necessarily apply in every combination of circumstances, which means that on Aquinas's account it is built into natural law that some kinds of polygamy are OK if the circumstances are just exactly right. On Scotus's account, dispensation arises from the fact that secondary principles of natural law presuppose certain conditions that depend entirely on the will of God. Thus Scotus can make the rare OK-ness of polygamy a matter extrinsic to natural law itself: polygamy is simply against natural law, it's just that God can make it so that the precepts in question don't apply to particular cases. This means that Scotism has a stronger position on the question than Thomism does; Scotus's full account of why polygamy is wrong would rule out Cajetan's view of Henry VIII, because Cajetan as a Thomist doesn't think divine revelation of the dispensation would be necessary, whereas Scotus's discussion at strongly suggests that he thinks it would be. But the Scotist position, while very slightly stronger than the Thomist position, is also a much weaker position than I imagine most people would suspect.
