A wife is acquired What does that mean?

Netziv (Meishiv Davar 4:35):Question: You asked a second time to discuss what does it mean that a man acquires (kinyan) a wife? Why and for what purpose is she acquired. This is that you wrote in the first letter in which you noted that according to the Torah a man makes no acquisition of the woman except in respect to intercourse – however aside from intercourse there is absolutely no acquisition. As a consequence if she makes a neder (vow) and says that she is prohibiting him from the pleasure of intercourse with her – there is no need to nullify the neder. But she can be pressured to have intercourse - in spite of the neder - because for that purpose she was acquired by her husband. Answer: This matter is very clear. This that it says in the Torah that the wife is kinyan kaspo (acquired with money) and also this that we find that the wife of a man is his slave and maidservant – the intent is clearly that she is like his slave and maidservant – but not literally so. Just as the work of a slave’s hand belongs to his master so is his wife regarding intercourse but not in any other aspect. A clear proof that a wife is not literally a slave to her husband is that our Sages say that according to the Torah, the work of her hands does not belong to her husband. But how do our Sages know this. Is it stated clearly in the Torah? But doesn't the Torah say that she is kinyan kaspo (acquired by money) which is the same description given for a slave and maidservant? So what is the source that her work does not belong to her husband? In fact let’s reverse the question, how do we know that his wife is obligated to him regarding intercourse and therefore cannot prohibit herself sexually to her husband. There is no problem if he was the one making the neder and said that he is prohibiting her from having intercourse with him. Of course the neder would not be valid because we have a clear Torah verse prohibiting him from diminishing her rights to sexual intercourse. And even according to the view that that verse is only talking about his obligation to cloth her, nevertheless the neder is still not valid because he is obligated to satisfy her sexually from a kal v’chomer. As we see in a braissa in the Mechilta (Shoftim). Rav Yonason said “she ‘era kesuba” is referring to clothing which is appropriate for her body. If she is young she should not be given clothing for an old person. Additionally that this verse can mean that she should not be given clothing for the summer in the winter and vice versa…. And how do we know that he needs to feed her?…How do we know intercourse?. There is a kal v’chomer. And those things which she didn’t get married for you cannot prevent her from having, those things for which she did marry to get she surely cannot be prevented from having. (I speak further about this in my sefer HaEmek She’ela (6:1). In contrast regarding the wife - she cannot withhold the pleasure of intercourse from the husband. So what is the source that says she is required to have intercourse with him? Perhaps it is from the fact that she is called “kinyan kaspo” (acquired with money) and that she is owned by the husband also in regard to everything else like his maidservant? But that is clearly not so and it is an elementary from the verse “When a man takes a wife”. Why does it end “And he has sexual relations with her” – and mentions nothing else? From this we learn that only for that particular aspect i.e., sexual intercourse she is acquired by him like a maidservant to serve him – but not for anything else….The kinyan (acquisition) of the man is only concerning the sexuality of wife. This is not a question regarding an unmarried woman according to the view of the Rambam who says it is prohibited to have intercourse with an unmarried woman. But even according to those who disagree with the Rambam – having intercourse with an unmarried women is only optional - but she is not obligated to have intercourse with him. And if he forces an unmarried woman to have intercourse – G-d forbid - then he is required to pay her for shame and degradation. Forcing an unmarried woman is like theft and like beating someone. In contrast his wife who is acquired by him – she is required to have intercourse with him any time he wants and if she does not do it willingly he is able to force her – just as a master who forces his maidservant to do who work. All of this is very clear and it isn’t worthwhile going over it again…. It is important to note that a man’s wife is acquired by him and also sanctified by him. It is important to understand that these two things are separate. Acquisition (kinyan) means that she is required to have intercourse with her husband just as a slave is required to do his work for his master. In contrast, kiddushin (sanctification) is like hekdesh i.e., she is prohibited to others. The significance of having two separate aspects is that from the point of view of the wife being acquired to her husband – it is considered theft if she gave her love to someone else and did nothing else. This would be like a slave who works for someone other than his master at a time when he had work to do for his master – this is pure theft. On the other hand purely from the point of view of acquisition, if her husband gave her permission to have intercourse with someone else it would be permitted – just as a slave who was permitted to work for someone else. Consequently that is why she is also sanctified (mekudeshesh) from which there is no escape except by receiving a Get from her husband or if he dies. However from the pure perspective of sanctification, I would not know that she has any obligation or that she is acquired by her husband. I would only know that she is prohibited to others through the sanctification. Consequently that is why she also has to be acquired. I have already written in the name of the Rambam that if one sanctifies an unborn baby that the kiddushin is valid and the baby is a married woman and is prohibited to others – but the baby is not also acquired by the husband. Consequently if a man sanctifies a woman who is prohibited to him by a negative commandment, she is definitely not acquired by him but she is in fact sanctified to him. Therefore anyone else who has intercourse with her is committing adultery…In summary, there is no doubt that a wife is only acquired (kinyan) by her husband concerning her sexuality and nothing else and there is no reason to repeat this again.