Ya, let's use Bella as an example as you and I are both on her route - right now, we are not in an exclusive relationship with Bella. Rumors are going around that we are, but we've never had a conversation with Bella about this as she basically ran and is now fortified in her bedroom in hiding.
So how would you react at this point (remember, not exclusive yet) if she comes back to school in a week or so, completely avoids the MC, but eventually you hear she's 'talking' with some other guy or is just seen around the halls with another dude who's walking her to classes, etc.? Would you have an emotional meltdown (it's okay if the answer is yes, I'm just asking, not judging or criticizing)? For me in that situation, I'd be offended but I wouldn't completely flip out just from her talking to another guy. Now if the character started getting sexual with another guy while we had something sort of going / developing (like we do with Bella now), I'd just write her off and look for another woman (would do the same thing IRL, too) - if I just saw her with another guy or heard she was talking with another dude / dating etc. I would just want a route in the game where I could have a good lane of communication to see what was up, especially after the result of the date we'd just had in an attempt to put the cards on the table so-to-speak.
To LadyFortuna's point - I see the same thing she described - in so many games if some (not all, but some) players so much as detect another male sniffing around the area their LI was in, they declare NTR and go nuts. So I do find it fascinating to see what other players are okay and not okay with. And in a story like this, where the developer is intending romantic relationship development as the main priority, having other guys involved sometimes really can help with that because when jealousy comes into play (as long as cheating doesn't occur) it can really resolve in a heartfelt way when two characters realize they really care about each other and handle things in a mature manner - Bella has already had a few instances where she gets irritated / jealous because the MC just mentions another female's name or goes and says hi to one - that can go both ways. If the LI is talking to another good looking guy and the player is a little jealous but the LI isn't really up to anything nefarious, just having an innocent chat (think Josy with Lucas in the cafeteria in Being A DIK and then expand it to be more repetitive in terms of interactions, possibly flirtatious if there is no exclusivity) that can drive additional emotional attachment in a good way.