I think it makes sense in two ways: Some people in right-wing world think that women are property, without individual agency. If another man has been able to have sex with her, you have failed as a Real Man™ to protect what's rightfully yours, in the same way as you have failed if a fox gets into the henhouse.

Or, a Real Man™ is so good at sex that his wife doesn't want anybody else. Either way, being a "cuck" means you're not a Real Man™.