It's likely a law that is still on the books but not enforced, at least not in the literal sense. There are lots of laws like that in RL that are politically, socially, and ethically incorrect, yet they remain.
The victor, Spock, won T'Pring and had the legal right to marry her, ignore her, take her away with him, or give her away (which he did). She then became the property of Stonn, as he is the person to whom Spock gave her. What Stonn chooses to do about the situation has been the subject of many fanfics, some of which have portrayed them as honestly being in love, some as T'Pring being no more than an opportunistic schemer no matter which man she ends up with, and there are some where Stonn chooses to treat T'Pring as legal chattel because she caused him to "lose face" at the marriage ceremony.