Rape is rape, I'm not going to defend it. The other stuff though, I wanted to say something about. The "forced" aspects of many stories solve a dilemma for those of us that insert ourselves into the characters, which is the shame aspect of it.
I remember someone once discussing why they liked abdl stories featuring robots malfunctioning and treating their charges as babies. The robots just plain don't know any better. To them, you are literally a baby. They're not someone playing pretend, they have the ample force and ability to treat you exactly like what they think you are, and while all of these stories are gonna be unrealistic, it's a story that helps let you, as the person inserting themselves into the protagonists role (usually), not feel guilty about being yourself (indirectly as the character!), letting it happen to you, it's relatively judgement free. You don't have to feel ashamed in situations that are outside your control. For many of us, in an ideal world, we could engage with our desires without any feelings of guilt or shame for being dependent on a caretaker, engaging in social taboos, and hell, it'd be a bonus to also be loved while doing it. There are many "globalized" fetish stories where this is exactly what's achieved.
So the forced aspect is a usually a guise by which we can let our own guards down as the self insert protagonists. There's another subtle element to that as well, which is that someone who is forcing us into something usually wants that thing themselves. Some of us want someone into diapers, want someone into being a caretaker. Someone who wants us dependent on them. Being forced by someone is of course an immediate solution to that desire. Not only are they someone who's fine with the thing we want, they'll even make us do it!
Circling around to the humiliation aspect. I think it's just a byproduct a lot of time. While there's absolutely a lot of folks who are into humiliation (exhibitionist sissies come to mind), I think the humiliation is something that just comes natural when we're trying to have a story that's grounded enough in reality for it to not break our own immersion. Globalized fetish stuff might break one person's immersion, magic might break yet another's, obviously for some here, being forced into it is immersion breaking (and also fucked up and illegal irl). But it's all relative to whoever is reading it really. Vanilla vs chocolate vs strawberry essentially.
What about the parental aspect? There's an element of convenience to it. Everybody's got a parent, and their parent is usually a trusted loved one who under most circumstances has our interests at heart. They're someone who's had natural control over us as children, control that by it's nature is socially acceptable. I don't think it's hard to imagine why parents would be a natural target for the antagonist of an ABDL story. Parents also offer a bit of the taste of what many of us would have possibly liked while growing up.
There's a theory in psychology I remember hearing, though it's name I forget. Basically, our subconscious knows what our desires are, and our subconscious also knows that bedroom behavior, sex stuff, is deeply intimate and usually only done in a "safe place" from a standpoint of relationships, socially, whatever. It's safer to act out our desires, at least typically. Fetishes from this theory are then byproducts of our subconscious mixing the sexual with our repressed desires/needs. I can't say how true this may or may not be. It's something I've only heard once before now and haven't heard since but it suits the conversation.
Anyways, this has dragged on for a bit but I hope it makes sense. Smut is smut, it doesn't have to entirely make sense at the end of the day after all, lizard brain go brrrr. I also say, as it's worth possibly reiterating, not everybody who writes this shit is actually intent on acting on it. There's creeps out there for sure, but it seems like we way over focus on them and try to attack far more people than is merited. Everybody here should be 18+ and know better. And with reading this, hopefully folks know even better than that now.