I happily read sex scenes, but in longer fics I need it to seem like THIS version of the characters doing the dirty (and even in pwp, the characters should sound like themselves). The kind of sex scene that makes me roll my eyes the most is the scene in any longfic where the characters stop seeming like the version of themselves the author has been writing this whole time, and instead seem to have been cast in PWP #4729462. Just, no. The sex scene should be tailored to the fic and the characters, and should both be informed by what came before, and add to what comes next. And sometimes, frankly, there are fics (chiefly on the plottier side, as opposed to the kind where the UST is the main event and the plot, if any, is more backgrounded) where any sex scene really is going to feel shoehorned in.

ETA: I also think the sex scenes in some plotty fics are TOO LONG. If what came before is 60k of UST, then maybe you need a 10k sex scene. If it's a longish PWP, then sure, make it any length you want. But if the romance has heretofore been the B plot, do things really need to screech to a halt for 10k? Probably not!