There are two options in the game settings, netorate and netorase.
Netorate is for LI cheating on MC with another male character. If you disable the option, the LI won't cheat but may still end the relationship under the same conditions.
Netorase is for sharing the LI with another male character. This is where the MC can instruct/suggest the LI engage in a sexual act with another male. For instance, to film the blackmailer in the act. If you disable the option, the choices for sharing will not be presented.
You either didn't play the gym scene (on Friday) or didn't pick the relevant option where Max discusses Kim not wearing panties.
Prerequisites not met were added because players wanted to know if another route would have been available. You can disable this in the game settings. It won't show the disabled option and automatically select the choice.
Did you play the Friday gym scene and notice the background characters in the yoga room?
Depends on the LI. Some don't require romance. For one of the male domination routes, the LI doesn't even need to like you.