Where was the option not to do that so they wouldn't break up?

Main/side girl is just a technical term. What matters is if the girl is just a slut or her and the mc being together is realistic which it is 'cause Bianca is LI material not (just) because of her looks.
Was it metioned when this project started that only Jaye/Mallory would have endings? If so, that info should've been put in the OP.
It's not about what I want, it's about what's realistic. Because rejecting/being rejected by all other girls besides Jaye/Mallory is not realistic. Not to mention that it happens at the end of the game too. Lisa for example should've told Alex that they would never be together instead of dragging him to that storage room and leading him to think it might lead to something. During the phone call in the previous chapter it looked like their relationship would progress not end...
In other games I always pick the main girl because yeah side girls don't have endings, but that's because usually they are nothing more than a one-time thing or they just want to "have some fun". Definitely not the case with Lisa or Bianca.