If it's not something I want done to me under any circumstance then it's not something that I think is ok to do to someone else under any circumstance. Lying and cheating aren't ever ok. It's the cowardly way out. Either you suck it up and stay faithful until you can talk about it or you talk about it and deal with the consequences...or you break up and pursue something else.
Everything will be ok in the end. If it's not ok it's not the end.