I know for a fact that favorite date doesn't always play a role as sometimes the NPC themselves will ask you before you guess their favorite date.
I myself am having trouble forming a polyculus. The first girl initially refused after asking herself to be official partners. I eventually asked her again and she agreed. The second one continues not to accept, although she has the dominant inclination so maybe she prefers to be my dom if she can't be my exclusive girlfriend? I honestly don't know since there is no real way to know what the NPCs want.
I'd like to suggest telling your dates that you're polyamorous or open or whatever from the get-go. Instead of everything being so strange, with them refusing first and accepting later, you would know if it is really worth pursuing someone or the type of initial relationship they want.
Nothing prevents a fuckbuddy from becoming an open partner or even a poly partner over time, but maybe that's not what they initially want. Same for someone single you're pursuing. Maybe they don't accept initially, but after several dates they are willing to try.
Yes, to some extent that is what is happening now, but at this point we have no way of knowing if anything will change. So if after the person initially tells us that he doesn't accept polyamory, whether we choose to continue pursuing them is up to us. Maybe they will finally accept us, maybe they won't, but that way if they say no from the beginning we can decide to pursue someone else.