Trying to write sex scenes is so aggravating because honestly describing sex is pretty boring. It's mostly just people putting bits of themselves on or inside other people's bits so you run into a lot of "they walked to another room" type problems where you can get caught up in just describing where everyone's limbs and shit are. What really makes smut interesting and hot, in my opinion at least, is sensory description but that's also hard because you'll be sitting there trying to find a new way to say "they were feeling sexual pleasure and it felt pretty good". Then you also have to figure out how to write dialogue that doesn't sound completely ridiculous and hackneyed. Really makes you want to just write "they boned down real good and it was totally hot trust me, it was definitely the kind of thing you'd want to jerk your shit to," and have done with it.