My understanding was that for progression with certain characters, it's non optional - I'll correct this after I play all paths.
I think you misunderstand the point of this game.
In my opinion, it is NOT about romancing all the girls. Instead, it is about romancing those that you like.
The girls have very different personalities. Some may be very sexually liberated, others may be quite suppressed. Some may fall easily for you, and some may need some, well, "heavy handed convincing" maybe. But maybe not, who knows at this point?
In any case, I am convinced that this game allows the player to pursue those girls he's interested in, while also allowing the player to leave the other girls alone.
Early on, there was a game that many considered NTR (which it wasn't, it was sharing). But what people forget is that that particular girl (like all the girls) is completely optional and if you don't like such "free spirited" women, then don't pursue them.