If a man is having a casual sexual relationship and he knows one of his plates is having sex with other men, that's a form of being cucked. I would call that guy a quasi or semi cuck. To many men, it does not feel good knowing that even one of your casual relationships is fuccking other men.
Technically, cheating/unfaithfulness can only occur in monogamous relationships. Based on that technicality, open relationship guys are not exactly literal cucks because of the framework of an open relationship. In reality, open relationship guys are quasi cucks too.
Another way that the word cuck is used is to describe men who are in committed relationships with women who had children with a different man. A childless man who commits to a woman with pre-existing children and doesn't have any kids with her is a cuck of the highest degree. He has a level of parental investment without any benefits of passing down his own lineage. A man with pre-existing children who commits to a woman with pre-existing children is also a cuck, but those blended family type relationships are somewhat expected and often an awkward fit.
Right now, with Boomers and older Gen X'ers, we're seeing a lot of 50+ men and women getting together who already have children who are 18+ and legal adults. Committing to a woman whose children are 18+ is also a form of cuckery albeit cuckery to a lesser degree. In blended families with adult children, parenting still has to happen. There are older couples who have disagreements about adult children in blended relationships. There are also some very uncomfortable holiday gatherings in these situations.
