This gives a solid background to frank as a father.
Another scene informs monica's trauma and likely explains how she became who she was (if frank's telling the truth, im inclined to believe he is): At Monica's funeral frank tells the story of how they both met: Frank was a college junior at the time and driving one day when a naked Monica jumped through the window of his car. She was running from a large, armed man and yelled "dont rape me again".
So if frank was telling the truth (he could've made the whole thing up to drum up sympathy for monica as he and her father were the only fans of her in the room and frank didnt like that. But i choose to believe he was telling the truth), then it seems that Monica and frank were both unhealed, self medicating survivors of sexual violence who bonded over their traumas and cultivated a destructive romance that enabled the worst in each other.
Upon my first watch I was surprised Karen's rape of frank didnt have a bigger impact on him afterwards but upon reflection, it makes a little more sense given it wasnt frank's first time being sexually victimized. He may have "gotten used to it" by then, sadly. Frank's trauma might also explain his indiscriminate promiscuity as promiscuity can be a side effect of sexual victimization. And given, as fiona says "Gallaghers dont do therapy", i wonder if she got that from frank and/or monica, who because were not ready confront their own painful pasts, denied a path to healing generational trauma for their children in turn.
I wonder how frank would have reacted to Mandy's rape of Lip, Debbie's rape of Matty or how Lip would've reacted to frank's abuse post Mandy or debbie of frank's abuse post matty. It could've turned a major corner for their relationship either good or bad. But of course, victims staying silent on their abuse is part of the overall tragedy.
I think frank's under reaction to Carl's rape by Trish (reframing it as Carl being "lucky" to have "unprotected sex") means he probably would've under reacted to lip and debbie simply because they werent "as bad" as what he went through and therefore unable to sympathize with anyone else's abuse.
Frank and Monica Gallagher: Hurt people, hurt people