I need serious help.. we don’t agree on what’s cheating

I never really thought this would ever be a problem with two people engaged, but I guess it is! My fiancé and I don’t see eye to eye about what’s considered cheating or not. He thinks cheating is only having sex with someone else or spending money on another person. I don’t think it’s that cut & dry. Cheating could be flirting, lusting after someone else, not being honest with your partner about your intentions, etc. I feel like I can’t trust him if he doesn’t understand that cheating is a spectrum, it isn’t just as simple as fucking another person. I don’t think I want to marry someone who thinks that way. How did I get this far without knowing he felt this way? What do I do from here