For ntr with more story rather than plain mind-breaking and the girls becoming addicted...
Some typical reason is the girl stops having expectation for the boyfriend to realise and help her. Just like the boys, they also don’t like to be the side who asks for help; such as afraid the boyfriend would find her needy and demanding, so they’d rather the boys notice, took initiative, and not put the blame on them especially if things doesn’t work out as expected.
Or in case where the boys wanting to keep the relationship “pure”.
Rather than “am I not attractive”, the heroine in those story seems to take it as their boyfriend is too afraid to cross the point of no return for commitment.
Like, the girl is okay with doing sex, but they expect that means the boyfriend express he is all-in for their relationship. But their boyfriend instead want excuse like “I don’t want to feel I owe you for taking something I can’t return”.