Make them all be helped by you

Pinkie's memory is be that she has never been able to get hard all her life, it made her extremely sad thinking of all the fun she was missing, not only that, but not being able to get hard made her slowly lose nearly all her friends, but you came and with hipnosis found a way to make her able to control her dick consciously, making her able to get hard or soft anytime she wants, this made her vow to be yours.

Sweetie's memory is that she has never been able to orgasm, no matter how hard she tried, it made her absurdly frustrated and a target for a lot of teasing, but then you came and used a rare potion to fix that for her, this made her vow to be yours.

Fluttershy's memory is that she has always had an erection, no matter how much she tried to make it go down, it made her already shy personality go into overdrive, making her a recluse, but them you came and with hipnosis found a way to make her able to control her dick consciously, making her able to get hard or soft anytime she wants, this made her vow to be yours.

Twilight's memory is that she is always orgasming, this wrecked her, she could never concentrate on her studies as she was always is an state of cumming, she was ostracized because of that and became a target for bullying, but then you came and used a rare potion to fix that for her, this made her vow to be yours.

All of them came from an Equestria in which females rule over futas, being a considerably smaller portion of the population it's normal to see a female have multiple futas under them, usually defective futas become outcasts, but if they get accepted those same defective futas become the most dedicated partners, now those same for defective futas have come to live with you following their vows.