Honestly, you miss so much content by going with one girl, I'd argue that the game isn't nearly as fun. I tried it a few times when I first found this game(right after 0.4 released), and while it was still fun, the knowledge of what I was missing by cutting off different girls' routes and how little the game caters to you sticking to a single girl, it just isn't worth it. I don't think there was anything more than a couple of changed lines to indicate that I had stuck with a single girl. The biggest change was the amount of stuff that I was cut off from. No Nancy, no blue-balled scene when the Eastern Roman Empire invades while you're having a bath. No Dalia, no blowjob on the beach(not much of a loss, since I'm not a big fan of that scene, but still). No Annie, no date in Andromeda, which is one of the best parts of the game.
And the longer the game has gone on, the more intertwined the harem has become. The girls all have their own wants and desires, and several are almost openly crushing on each other. Sooner or later, you're going to miss out on content with the one girl you've chosen simply because you've cut off content with one of the others. Penny and Nova, at this point, are practically guaranteed a threesome scene at some point. If you're not following the path of either one, you're losing out on that content. Compound that with 7 mainline harem members, plus Calypso, and that's a lot of missed content coming up.
As much as I respect that people have different tastes and are totally allowed to like/dislike whatever they want to, this game was designed as a harem-first game. That's the primary way it's meant to be played. Wishing or asking otherwise is like someone walking out of a Godzilla movie and saying "I liked it, but golly I wish it hadn't been about a giant kaiju stomping around a metropolis wrecking everything." I'm not saying that that's what YOU are doing. You were just asking a question out of curiosity, I know. But I've seen a few people here post that kind of notion, and it doesn't make sense to me.
