By withholding sex, a partner is violating the purpose of making the relationship exclusive; passively injuring the mental health of the other person. It's ridiculous to deny your partner sex and then act all surprised and outraged if they have sex with somebody else.
The person in the relationship with the lower libido controls the relationship while the other experiences incredible loneliness and feelings of rejection. If two people are living together and not having sex… they're roommates.
When we accept a person's assurance of monogamy, we take on a duty. How can one expect monogamy and refuse sex??? It's like saying, "Now, you only have the right to shop at this 'one' store. By the way, it's going to be closed 100% of the time. They're not out of business, they're just never open."
A wife that denies her husband is breaking the wedding vows just as much as if she were cheating.