She is, the 3 mains are Monica, Katie and Jenna. All the other girls are sides. And out of 3 mains, Monica is the Main one. BTW I'm a Kaite guy, so I'm not some Monica fanboy. It's just I understand the structure of stories.
The first girl we meet? Monica, whenever I see anything about this game outside of here, the "Cover" art if you will, it's Monica. Who's story is most up in the air out of the 3? Monica. Who have we still not banged out of the 3? Monica.
Even stories that are ensembles have main people in them.
The main love interest is whomever you want it to be. You yourself said that there are three "main" LIs. Therefore it's a choice. But there are also other options. "Main" is subjective. If you can have an ending with anyone, then they can be a main LI to you. Sure, Monica might be the one with the classic focus of "wait longer, build suspense, bang last", but that does not mean that she is the "main" LI. Plenty of games have a "main LI" by your definition (save for the long wait) where you can have sex with them in the very first release. Monica is the MILF character. That's why she gets the most wait time. Because a majority of people on this site have the MILF fetish, and developers know this. That's why they make them wait. To keep interest. It's a tried and true trope and method that you can see in almost 90% of incest games (or games with a similar "familial" plot, such as this). Personally, my "main" LI isn't one of the three mains, according to you. Monica might be the one with the MILF type of focus (and to be fair, those characters
typically are the main LI, but not always), but she doesn't have to be the
primary focus. That's why exists an option to not pursue a romantic relationship with her. Just like Katie and Jenna. Do they get the most story? Yes. But they can have story and not even be a love interest. Are they meant to be the ones you fall in love with most? Yes. Does that mean you have to? No. And that alone means that they're not "mains". In this case, anyone can be a main based on the player's preference. It's all what you choose.