To not have the wife exception. Previously the idea of 'raping your wife' made about as much sense as 'stealing your own car' - it's your property, you can do whatever you want to it.
The idea that wives were ever property of their husbands in anything other than a very weird technical sense is misguided, and derives from an obsessive over-focus on certain classical societies, as well as a misreading of the doctrine of feme covert. Most of our modern family law, and indeed English family law going back before the Conquest, was derived from the Germanic law. There, wives were considered persons (everyone's a person, how could they not be) and could manage affairs independently. This combined after the Conquest with the Roman law system, where wives were still persons in all real senses but came under the protection of their husband, which restricted their independent authority but meant the husband was obliged to follow their wishes in doing most forms of business using their wealth. This was reduced a bit when trusts were suppressed by the common law in the 14th and 15th centuries, but came back hard when the Courts of Chancery resurrected that institution.
Through to the nineteenth century, women's legal personhood was in full operation, and even a married woman could generally act without impediment outside of the actual marital relationship. The marital rape exemption doesn't come from the idea that the woman is the man's property, but rather from the general unwillingness of the law to intervene in the family sphere; people like Blackstone quite explicitly talk about it as something that's obviously very bad, but not the law's place to stop, and in any case stopping it would involve such a destruction of the family's independence that it wouldn't be worth it. Obviously we now don't have much time for that argument, and the family seems to have survived just fine now we punish domestic violence, but that was the argument they sincerely believed.
The women-were-property stuff is just hopelessly wrong. If you're interested in the topic, I can recommend some books.