I'll take a different tack on this. Read My Answer here that goes into the Three Act Structure in some detail.
Basically, you increase erotic tension the same way you increase any story tension: What you need is a good story that demands intercourse in order for the protagonist to get what they want. Now, in all my answers (and work) my MC are females (unless the questioner is specifically talking about a male) so don't take the following any differently because the subject is sex.
In the 3AS, we introduce the MC and show her in her normal world, solving normal every day problems. At about 1/8 of the way through the story, there is an "inciting incident" that, by 1/4 of the way through they story, develops into a big enough problem that the MC must leave her normal world (the status quo won't work anymore) and embark on a mission to solve the problem. You can read the rest of the structure at the link.
What you need to do is write a good story, that keeps the reader wanting to read what happens next to the MC, a story in which the reader likes the MC and wants her to succeed, a story in which the MC is being denied what she wants or needs but keeps trying to achieve it, until the obstacles are overcome and she does. After that, in the last 5% to 10% of the story, she enters a new "stable state" that is either her old status quo, or some "new normal" for her.
That is the story arc.
What you need to do is make your story problem about something erotic and sexual, so that our hero's attempts to solve her problem involve having sex which you can describe in as much detail as you like. So her adventures are not battles, or infiltrating castles or figuring out what evil plan the protagonist is pursuing, but are all sexual adventures of various kinds.
I can think of many inciting incidents, from real life, that might make an MC (of either gender) become obsessed with finding sexual gratification, particularly in the modern world. I think men need less motivation in that respect than women.
For women, I knew a woman in college that truly believed she was a Wiccan, and she could cast a powerful spell during an orgasm with a new partner. A nice weekend hobby, if it isn't finals week. Or maybe especially if its finals week, I'm not sure about that. Or a breakup (especially if there was cheating involved) may cause a kind of revenge promiscuity. Or, again from RL, from a study on cheating spouses, I know OF a middle-aged woman tasked with caring for a husband severely disabled in a car accident. He was incapable of sexual activity; and after a few years of this, for her own mental health, she began (through social media) engaging in non-love sexual affairs, specifically never with the same person twice, because she had no desire to leave or divorce her husband, but felt she needed a kind of "I'm still living" break from being a constant care-giver. Other than the Wiccan, it might be hard to develop a good Three Act story from those motivations, my only point is to show such motivations exist.
So I'm not telling you what to write, that is outside the scope of this site: I'm telling you how to write. Think up all the sex acts you want to write about, and perhaps arrange them in the order of how intense you think you can make them. Use that order in your arc of increasing sexual tension.
Figure out what the ending looks like, for the MC, what would the "new normal" actually BE?
Then the increasing sexual tension comes from the normally increasing tension of any good story. In the first half of Act I, we see your MC engage in explicit sex, but the least of intensities for your scenes. An inciting incident (catalyst incident) occurs, and we have another sex scene in the 2nd half of Act I, that one causes the MC to leave her "normal" world and start her quest for ever more intense orgasms or whatever. But she IS seeking something that she believes is out there and she is NOT finding it.
You have to keep the reader wanting to know what happens next, not just in terms of sex, but in terms of whether the MC is going to succeed.
Write a STORY in which the MC is seeking to find something or accomplish something or learn something or solve a mystery with sex. Make that difficult for her, even if she does orgasm, she doesn't find what she was looking for (until the literary climax). But after the first half of the book, she should probably get closer and closer to it.