Yeah, but all those are optional, so he has a point.
You could say that the MC had fantasies about being with other women, but even his fantasies are optional. The practical reason why you can't choose to stay with Fiona and Fiona alone is because it would branch into its own route, which would add too much workload. Slower updates, longer dev times, later release, and more sleepless nights for the poor dev who makes the game.
Still, I tried to make it believable. MC is basically another character. The game is a harem game. There is no real faithful route, even though you can decline any sex scene in the game.
You can choose to be a moral person and not fuck any of the girls until Fiona gives her okay but declining all the girls would basically shorten the game to 1/13 of it's original content. If you choose to do so, then you can also stay abstinent after Fiona gave her okay. But that's not how I intended the game to be played. You could argue that a good game would allow for that sort of path... But I decided against that. It's a 1 MC 13 girl harem game where you can choose not to have sex with any of the girls you don't like.
About Megan going bowling