Love requires certain events like logan he only gets one heart and that is when you save him instead of bernard however with bernard he gets a couple hearts from different events. Which also means please be more specific to who you want a heart with, unless it's literally everyone then I'm gonna try my damndest to help but no promises.
Oh also I almost forgot you said everyone! (Well I kinda did so sorry....) And for that answer there will be a certain requirement to see if they can have a love route/sex route. That would happen to be if you can talk with them and they have a like meter in the menu, which basically everyone has you talk to that has a name does. Meaning almost everyone encounterable so far will most likely get a route for love or sex.
(Excluding enemy like meter, it has to be npc like meter. This doesn't mean that bosses you fight will be without a route though because of some bosses you encounter in the beginning and can talk with more later such as, Roushk, Scar, tank and gunnar.)