How To Avoid Adultery In Your Marriage The Bible is very clear that adultery is sinful. Adultery is when the covenant of marriage is broken by fornication and lust. If you’re currently married, you must not engage in any sexual relationship. And you must not cheat on each other with anyone. On the other hand, If you’re not married, you must not engage in any sexual relationship with anyone who isn’t your wife or husband – if you do, that