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. (对恋人或配偶) 不忠诚的

James had been unfaithful to Christine for the entire four years they'd been together. 詹姆斯在与克里斯蒂娜一起的整整4年里一直是不忠的。