Look, I don't want to get into a debate over what constitutes okay-abuse and not-okay-abuse in fiction, but I'll bite. Whether or not you take it out in the future, the facts remain that 1) it's still in there despite having little contribution to plot/characters, but more that 2) at one point, you sat down, asked yourself how you wanted to build flaws into Jimmy's character, and said "hey I know let's have him casually beat his girlfriend." Even ignoring the fact that this incident only exists for one chapter and no one brings it up again, the entire narrative lens here is focused on Jimmy. You hardly acknowledge that, hey, there was an actual victim and it was Flannery; rather, you tell us we should feel bad for Jimmy because he feels bad and basically everyone else in the situation has moved on but hey Jimmy. I say it again: you're perfectly welcome to include whatever narratives you want in your story, but mature things like domestic violence shouldn't be something you just throw in there to "replace" when you decide it's awful. If you're going to include mature themes, you should 1) understand them well enough to write them realistically and 2) address the actual implications that these themes would entail. This extends well beyond the domestic violence stuff--a lot of your more mature aspects seem thrown into this otherwise child-friendly plot.
I don't want to sound rude. I work 8-6 during the summer. I'm prepping for a pretty rough year of college. I want to maintain some semblance of a social life. I also have my own writing projects and follow a bunch of other stories on this site. I read approximately a small novel's-worth of your story over the weekend because I had free time, and I told you my thoughts there, but if you're honestly going to go with "the first something-thousand words are rough but the rest is awesome," I don't know if I can convey how little time I actually have to read the first something-thousand words to get to the good stuff.
I mean, seriously, you do you. You can write whatever you want, and while it's in my power as a reader not to like your story, that should not have absolute say over what your story is about. If you want the sex scene, have the sex scene; I was just saying that it wasn't my cup of tea/could've appreciated a more explicit warning tag. Mostly just the warning tag, 'cause I don't know if we're used to having graphic content here. The rest of the stuff--aversion to the fact that literally the entire female cast has had at least one instance of casually enjoying bondage/submission, the brushing aside of the domestic violence, and the kind of awkwardly out-of-place sex scene--are mostly personal thing that literally only apply to my reading experience. If you want to write that, please, don't let me stop you. That sounds really salty in text, but it's not meant to be. It's your story.
That being said, I'm pretty sure the US internet law for porn is still 18, regardless of state-by-state age of consent. It's more of a research thing, though--when you write about things that you don't know about, be it sex or drinking or, idk, spaceships, if you don't know something, it'll show. It'll look weird. And with a scene so detailed, it's like trying to write a detailed step-by-step account of how Jimmy built a car when you've maybe only ridden one a few times but haven't actually constructed. From a technical, strictly-observing standpoint, it just looks odd, I guess.
That being said, I can't stress the top paragraph enough. It's still your story. I give comments and suggestions, and I wanted to let you know why I felt uncomfortable reading the rest of the work, but don't take my reviews (or anyone else's) as absolute law.