wait I got kinda confused is she saying she's infertile or did red head just use it as an insult? ┑( ̄Д  ̄)┍
Basically, Allen used to neg her. He would be all nice and sweet to get her attached to him and in the mood. After having sex, he insult her and make her feel bad so that she doesn't go for someone else.
It's something people do when they want their partner to feel like no one else would want them. "I like you but you are very flawed and no one else will want you but me." In actuality, if someone like Allen wanted Daisy then she could probably get someone else to like her.
She's not infertile, she could have miscarried without being affected from really early term when there isn't much from all the stress she'd been under. Stress can really mess with your whole body chemically by making you ill, causing bodily functions to go astray, or even harming you mentally.
If a female is pushed past a stress limit it can send the reproductive system out of whack making them irregular [egg isn't released], constant menstruation for months on end [that one is the worst T-T], can cause cysts [worse when those get agitated to the point of bursting rocking many organs with it], or possibly causing other potential issues. Stress can do a lot to the body.
If you add the emotional abuse Daisy was under you can see why she would be considered infertile since she could have been experiencing irregular periods. Just my tid bit of information on that take.
Like Hunterx2 mentioned, Allen was most likely just abusing/gaslighting her only which doesn't help the MC's [re-existing insecurities.
Usually when the questionable Infertile trope gets thrown around in these stories, the MC usually is in fact fertile and typically ends up with kids anyways
Heck, and in some of these cases, it's the guy all along who was shooting blanks ╮( ̄▽ ̄)╭
I'm leaning towards the author using real logic there so we can hope she has a happier life this time since it seems like he'll treat her right. Plus us knowing they hit every flag possible for the love route we can expect this to be a very entertaining love story it's just a matter of who falls first lol.
T_T ┗( T﹏T )┛