Well, I'm not going to get into a long argument about it but in most cultures throughout history, marriage is a commitment between a man and a woman sealed by sexual intercourse. In most marriage vows there is a section where the parties give themselves to the other unconditionally. This includes sexually. It also means that in the marriage contract, the starting point is that consent to sex has been given because that's what the vow says. The change in the law which was made in the 1990s cuts across that and was designed to deal with the situation where the marriage is in difficulties and essentially that consent had been withdrawn.
Logically, there is a difficulty in having made a vow to consent to something and then saying that you have to consent to each and every individual instance. Either you've consented or you haven't - our current law confuses the issue. I agree that consent can be withdrawn but I don't agree that in marriage, consent has to be given in each individual instance. Most cultures would hold that such an arrangement is not marriage.
Mind you to be honest, most people's views of what actually constitutes "marriage" bears no relationship to cultural, historical or legal reality at all.