clearly they think they can handle it or are willing to take what they can get but their emotions get the best of them

this is like if a girl really likes a guy and wants to be wifed and he's not with that. even though he's straight up telling her it's never gonna happen. it's not nice or fun but you gotta put the hammer down on shit like that because in the situation you described, the girl is the only one thinking/acting clearly. at that point, you have to euthanize the shit. looks like a perfectly good relationship but it's not. dude can't handle it and can't control wanting more.