Mostly is a matter of story. Unlike most VNs with optional love interests where each lives in their own, pretty much isolated compartment of the game, with their own story branch more or less separate from that of the other characters, in this case their story lines are very much intertwined, and they play and bounce off each other to drive development. It just doesn't make sense to have one or the other be optional in there.
There's a technical aspect too: if I'd wanted to offer such an option with three branches (which I don't, for the record), I'd have to write two completely new branches for the story, mutually exclusive with each other and the current main story branch, to facilitate that. Then I'd have to come up with all kinds of events and motivations to make things work with one or the other not having any skin in the game so to say.
That is is an enormous amount of effort in it's own, and reduces my efficiency by two thirds. I'd have to put in three times the work for any update, but for any individual player, the amount of content in an update doesn't increase, as they are only able to see one third of my effort.
Obviously, that is far from ideal, as the amount of work it takes to produce this stuff is already ridiculous.
Finally, for the record, Macy and Denise as a whole are an avoidable branch, that is, the MC doesn't have to get involved with them. If he doesn't, they'll just get on with things by themselves. From a technical point of view, this is a lot simpler to implement (the plot mostly stays the same, minus the erotic parts that that involves the MC and them, although there is some effort in alternative dialog to reflect this and turning any dramatic developments involving the MC platonic).
" and leave me alone creature 
