As an editor, I've heard several authors who were writing erotica or erotic romance or just straight up romance ask me the all-important question, 'Do you think there's too many sex scenes in my book?' While I will say there is definitely a point at which some reader somewhere might be making her way through your book and think, 'Wow, enough is enough already,' I just wish I would hear just one author one time ask me the way more important question of, 'Do you think I need to add more sex?'
The only fatal problem I've ever found in the areas of erotica or erotic romance with regard to how much sex is in the book is 'not enough.' I slogged my way through one terribly written cliche after cliche novel recently that was supposed to be erotic suspense, and at the end I realized that in about 250 pages there were 2 sexual situations, and one of them was a blowjob.
I've heard so many people try to explain it a certain way. Answers I've heard range from 'every other chapter' to 'every character at least one' to 'every X number of pages.' I actually like using that last one just as a writer's check in revising, but I do not profess it as an overarching rule. Yes, I will suggest to a writer, go back and read through your entire book and mark the frequency of sexual situations, but overall, I hope no one approaches the telling of a story with a strict measuring stick for the juicy parts. Sometimes one sexual scene will actually lead straight into another one, whether by effect of the plotline or placement of characters or whatever the reason. Other times you will need to further your story along a little bit in order to get it told. I mean, if you have your heroine lost in the woods all alone, not knowing if she'll ever get out or get found, you can't really throw in a sex scene. Unless you're going to go with masturbation, dream, or flashback, but those are really really hocky constructs if there's no plot reason for them.
So I hope that's enough contradiction for you today. Ever notice how writing advice never gives you a clear answer and tends to contradict itself? Don't plan your sex in a per-page way, but when you finish writing your manuscript, go through and see for yourself how often the sex happens. I know, I"m not very much help. So let me just leave you with an old adage and a new one.