Seems like you are the one that don't get it. The way he explained it. If the MC starts having sex with 1 or 2 of the girls what are the others going to do? Basicly saying they should have fun with each other because MC is busy. I say why they got to be there when MC is with the other girl. Like their whole lives revolves around sex and if they see someone else having sex they going to go into a frenzy and fuck each other? Like example 6 girls is in the living room watching tv, the MC walks in and grabs 1 of the girls and lays her down on the couch and starts to have sex, and the other 5 girls look at each other and say "Hey lets have sex with each other while MC is busy with her" That seems to be what he was saying. But for me in that senario the other 5 girls would do 1 of 2 things 1 get up and leave or 2 tell them to go to the bedroom if your going to have sex.
You don't get it, let me try to explain better: if the MC is in a relationship with 4, 5, 6 girls, and they start an orgy, IT'S FUCKING NORMAL to fuck between eachother! And this is only possible if the girls are bi (or if they are porn-actresses, but this is not the case). What is not normal is that if the MC is busy (during an orgy) with two girls, the others wait their turn without doing anything, at least they should masturbate in the meantime.
In this particular case, the whole VN evolves around a poly relationship, and you can't have a poly relationship if the girls are not bi. In fact, both the first two MC's girlfriends are bi and they fuck between eachother; then there are the "sisters" that are able to join the fun only because they started to fool around between themselves a bit first.
I've never mentioned a situation where the MC barges in and start mindlessly to fuck a girl.
That aside, this VN is made this way, you may like it or not, but you can't force the dev to change the story because someone doesn't like lesbian stuff.