To summarize:
Creator logic: "The characters are bi, but they are in a male x male story. Nothing changes for the player."
Hater logic: "If you label them as bi, you are opening the door for women / non M/M fans to join the community, and eventually, the game will change cater to them."
My opinion:
I think it's rooted in long-time supporters who've enjoyed this vn cuz it's already a specific niche for years, which is M/M (it's like their safe space), and now that you're rebranding, ppl will start to protect it in good or bad ways.
Easy to understand is like when a creator changes the character sprites cuz it looks better; some ppl still prefer the old ones, and in your situation, it just makes ppl take it to extremes.
I think ppl like that are concerned that a broader demographic also means the gay-focused story will be diluted.
Are you sure that, as a creator, if in the future you add more female characters, you will still keep the same quality, time release and M/M focused the same or it will start to shift to M/F?
You are also removing the Gay tag also a bad choice. This tag is to tell that everyone can play, but the story will be about men loving men. When you remove it, that means M/M is not the main focus anymore, and new ppl could demand the creator to add M/F content, which is what the og fans are afraid of.
The moment you take ppl money for this project, you have to treat it like a business, not a hobby. So not only are the creator's opinions important, the players and the fans are too, and even the haters (haters - ppl that used to be your fan). And from that, you will have to make choices that make the most sense for all the ppl, not just you.
Also, trying to argue with the haters is just like adding fuel to the fire, and for you as a creator, it never ends well.
About the diversity, there is nothing wrong with having more diversity in a VN, but at the same time, it doesn't need to have it all in one. Being the game itself rn, a M/M furry VN, is already a specific niche to contribute to the VN community in general. So whether you pick, you have to be smart and logical too.
Solution:
The easiest way is to follow where the money goes, make a poll on the P platform, ask if they want to add smth new like female or bi male char, etc. If the majority agrees, then do it; if not, then don't. It's not worth it to go against the ppl that paid you. Everything else didn't matter much.
I'm 100% being neutral abt this situation here, and curious what will come out after all this drama.