unfaithful

1. ADJ If someone is unfaithful to their lover or to the person they are married to, they have a sexual relationship with someone else. (對戀人或配偶) 不忠誠的

James had been unfaithful to Christine for the entire four years they'd been together. 詹姆斯在與尅裡斯蒂娜一起的整整4年裡一直是不忠的。