Lemons are far more difficult to write than a lot of people seem to give them credit for. Firstly, you need to write a plausible sex scene in an anatomical sense, and a lot of young writers do fail at this hurdle, though for obvious reasons, I'm assuming. Secondly, it must make sense in terms of the plot (having a random sex scene for no reason tends to just cause mood whiplash rather than any actual arousement). Thirdly, and perhaps most importantly, it must be in character. If character A would not plausibly be doing that with character B without any sort of build up or explanation then it doesn't work.
I'm actually trying to write one myself for a kink meme. Although I'm not failing sex-ed, I'm finding it difficult to both make it plausibly in character as well as potentially arousing. I'm actually on a second draft because the first was more in character on both sides but was taking far too long for a single sex-scene.
Most original erotica I've read has had issues, too. It seems to be that many writers think that it doesn't have to be as well written as other kinds of scene because a description of sex should be enough. Which isn't the case.
A sex scene should have as much attention as a fight scene, a death scene, a conversation in a cafe, a description of a walk into town or any kind of scene, really.