Boundary vs. the manual

Hey lovely coach: I usually think I understand the difference, and then I need clarity. I understand that boundaries are set to keep our own emotional wellness. We set a boundary in regard to violations of “physical and personal space.” Examples that I have heard and read seem pretty straight forward: people smoking in your car, showing up at the house unannounced, or yelling at you or hitting you. Got it. The baffler for me is cheating. I understand that I may tell my SO that I have a boundary regarding him having sex with another woman or looking at porn. My question is how is that a boundary? Isn’t that a manual? If he cheats or looks at porn, that is not invading my space. And people who cheat don’t usually tell the other person that they are cheating. So I would not know the boundary is violated. Thanks for any insight.