snogandagrope answered your question: Omegaverse Headcanon Question Time!

This bit: He enquires into John’s experiences of heat in graphic detail…exploring what…they liked, what they’ve fantasized about.

Ohhh!  Yeah, I kind of wanted to expand on that, but it felt like this big fat infodump in the middle of the action, so I nixed it. XD

Since I don’t have a word limit, the full text of that passage is:

He enquires into John’s experiences of heat in graphic detail, absorbing every scrap of information about what happens to his body, his mind, his emotions, and what he thinks of it all, along with what John knows of alpha heat and what Sherlock can expect once he encounters John’s pheromones. He probes into the intimacies of John’s sexual exploits and shares his own with equal nonchalance, exploring what they’ve tried, what they liked, what they’ve fantasized about. 

Wow, I really crammed a lot into that bit, didn’t I?  I’ll have to make a note of that.

But to unpack this, this is the bit where I gloss over how these two spent THREE MONTHS talking about sex—how it works for them as individuals, their opinions, their hangups, their personal preferences and sexual histories, and negotiating what they want from this and what they want from each other, and planning out how the entire nine-day encounter is going to go.

First of all, in my omegaverse, alpha heat (colloquially and rather pornily known as ‘rutting’) is a physiological state in its own right.  Triggered when an alpha gets a good dose of an omega in heat, a cascade of hormones then begins in the alpha, preparing him for, y’know, a marathon bout of sex.  Because under normal circumstances, a human male is NOT prepared to indulge in several days of multi-orgasmic goodtiems. ^_^  If the alpha removes himself from the vicinity of an omega in heat, the rut eventually winds itself down, over the course of about 24 hours.  (But during that time, he remains prepared to nail anything that shoots him a come-hither look, and light into any other alpha who looks like a possible rival.  Alphas in this state are tricky and difficult to deal with.)

Heat and rut both induce sweeping physiological changes, in preparation for what the body is expecting to happen.  What the person can expect is:  intense and extended sexual arousal, the capacity for multiple orgasms, elevated body temperature since the metabolism does some wonky shifting-around (especially for the omega).  Heightened senses and easily stimulated nerves as a by-product of the whole chemical cocktail.  There’s a periodic adrenal surge which, combined with the increase in testosterone, makes alphas in particular more aggressive, but it makes the omega’s mood pretty volatile as well (the omega’s testosterone actually decreases from their normal levels—which are low for men but high for women—while their estrogen count increases, in preparation for possible conception).  And, of course, enough oxytocin to float an ocean liner on.  Heat and rutting make the monthly hormone fluctuations of your average menstrual cycle look calm and laid back.

So that’s all an issue in itself.  And of course, as is the way of all things re: body chemistry, this is a generalized template but every individual interacts with it differently.  There are even related medical conditions where, for example, it can trigger a heat-specific round of bipolar disorder, or adrenaline-based ‘berserker’ rage.

Sherlock has had sex, but he has never gone into rut.  Which is a Big Deal in practical terms (some alphas would look down on him for it too, not that he or John give a damn), because this is a lot like pregnancy, in the sense that it’s a natural but complex, fairly high-impact thing that gets a lot easier once you’ve gotten familiar with how YOU do it.

John can help him somewhat, since he has some familiarity with the condition from, y’know, sleeping with his fair share of alphas during heat, but of course there are limits.  He can only say what he’s been told about what it feels like, for example.  He can’t tell Sherlock all about what it’ll do to him in particular.

(So it’s worth noting that they were taking certain chances with this, like the low but entirely real possibility that Sherlock could’ve turned out to have some kind of heat-related mood disorder.)

Also, no, John isn’t entirely comfortable with everything that happens to his body during heat.  It’s…a bit alarming, honestly, to have control of himself taken away from him like that.  It’s exhilerating and yeah, it feels really damn good—and if he’s with someone he likes and trusts enough to let go and just enjoy it with, it’s AWESOME on multiple levels—but it never stops being kind of frightening too—not least because if something goes wrong, he isn’t in full control of himself or his faculties to deal with it.

😀  They so made some very practical arrangements.  I had to cut it because it didn’t fit, but originally I had a section about how John had cleaned off the flat surfaces, pre-cooked and boxed some meals for reheating in the fridge, rearranged the furniture for minimal tripping-over, laid in first aid supplies in strategic places in the flat, and some other stuff.  He’s done this enough times to know how to make it as convenient as possible. ^_^