Making up lies, will just end up making you a fool. You'd be a fool to cheat on your spouse, eventually you'll get caught. In either situation; you're a fool
Why do either?
Lying about anything is wrong. So either way, it's not morally right.
I say grow up, quit playing chilish games, if you dont like the person you are with dump them.
Making up lies about a fake relationship sounds totally pointless and could never be as bad as actually cheating, because nothing has actually happened.