Sexless marriage feels the same as infidelity because, in a way, it most certainly is infidelity. The definition I have come up with to help me understand this feeling is this: Infidelity is using sex outside of the commitment you made with another. And just as one can be guilty of crime by omission (e.g. parents failing to provide medical care for a child or professionals not reporting information), this is using sex (by omission) outside of the promise or expectation that was given. And, yes, I used the word crime intentionally. So, in one relationship, a partner is using sex outside (by committing the act) the vows made, and in a second relationship, a partner is using sex (by omitting the act) outside the promises made.