This kind of infidelity (netorare) is orchestrated by the deliberate actions (or inaction) of the protagonist or the main character. Only one member of the couple have sex with others.
The heroine wouldn't willingly cheat on the protagonist, but he intentionally manipulates the situation so that she will end up raped by other men, or just flat out convinces/orders her to have sex with them. Either he does this for personal gain (selling his girlfriend for money or using her to obtain blackmail materials on someone), wants to push the heroine away, or he enjoys such a situation where the heroine has sex with other men.
Same logic if it's the boyfriend or husband who have sex with other womens for the "beneficts" of his girlfriend or wife.
Don't misunderstood with Swinging where both the boyfriend and girlfriend have sex with others.