Actual penetration is probably the hardest part of erotica to write about. You can go on for pages describing seduction and foreplay, and the delecate act of undressing, but when it comes down to penis-in-vagina/ass sex there often doesn't seem to be much to say. You can only use the verb "thrust" so many times after all. You can do different positions, but then it ends up seeming like a porno. This is, I think, why so much smut ends in an anticlimax, with a few throwaway paragraphs following a whole story's worth of build-up.
I'm not sure I've found the solution to this problem yet, and I'm sure having trouble with it as I write the next chapter of Saying Stay (coming soon!). I could go for a lot of implausible dirty talk, but that's not really my style. Instead I'll try to veer between crudely physical ("He thrust his cock into her cunt faster") and overly metaphysical ("Their bodies and souls melted together in an act of passion"). If I fail, at least I won't be the first.