If you mean meeting Iris in the first place, you'll have to reload a save unless you haven't done Lily's taboo break event where you buy her a new phone. If you've already met Iris, then you don't need to worry about further progressing her storyline, because the game doesn't have any further story progress for her.
There's still the business management aspect, if you like that part of it. It can be fun to just try to make a lot of money, but it gets pretty easy after a while. Other than that, I don't think there's much point, unless you really enjoy the wardrobe customization elements this game has (or light transformation elements with weight and breast size). Off the top of my head, I can't think of any aspect of the game that rewards you for monogamy.
Okay, that's good. Can you tell me what event you were thinking of, so I can look out for it?