I have never cheated in a LTR or when engaged because it was easier for me to transition to friends when it no longer worked rather than prolong the pain of basic incompatibility.
If I married and the spouse withheld sex or we grew to be sexually incompatible - I would have sex with other people. As I do not lie, he would know and be fully informed about any partners.
He would have the option to leave if he wanted, he could have sex with others if desired, but I do not believe infidelity or outside sexual encounters are enough reason to leave a good marriage/relationship. Certainly on my end infidelity would not irretrievably break the trust, love, or respect I'd hold for the mate.