My point is that you said, "so I'm not sure where his right to have sex with someone who consents, comes into this, as apparently she can't, and how, in these circumstances can she be (or others) who are vulnerable, be protected within a marriage?" and I was trying to explain in simple terms about what the law means by 'fundamental right' and how this case will attempt to establish how individuals who don't have the capacity to consent to sex can be protected as per the MCA.
And yes men and women are and have been prevented from having consensual sex if they have a learning disability, dementia or a mental health problem for example, despite them having the capacity to make this decision. People have been moved away from their partners, women have been sterilised without their consent and men have been medicated to reduce their natural sexual drive.
That's why the MCA is an important piece of legislation, and it's vital that the practical application of its statutes are considered in court.