How hot is too hot? It depends on your target audience, the genre requirements and your own comfort levels. Don’t try writing explicit BDSM if it makes you cringe, as this will come across. Equally, if you feel your scene is too tame, don’t be scared to spice it up a little.
As always; read. All genres have their own expectations that come with them and it’s up to you if you choose to write safely within them or skate close to the edge. Read authors in your genre that you admire and take note of their self-defined boundaries. Also, listen to your own story and your characters and take a look at the setting. As always, consider your plot. Ditto with your genre. If your novel has a historical setting, consider the social mores of the time. Upper class ladies in Regency England are highly unlikely to have been engaging in same sex threesomes, but go back to pagan Rome and you might be on to something. If your thriller has an action packed plot then anything but the simplest of sex scenes may be unnecessary and distracting. Don’t be afraid to include purely ‘vanilla’ (no toys, props, dressing up or Kama Sutra positions) sex. It can be surprisingly hot. A long, slow afternoon in bed or a passionate quickie outside, if well written, is often all your reader needs and as a general guideline if you’re writing in a genre other than erotica or hot erotic romance then keeping it simple is usually best.
If you are writing erotica or erotic romance, there are clear categories for heat levels and explicit scenarios within most sub-genres, so your story will fit in somewhere but as above, think about your plot and characters. Rather than, say, throwing in an anal sex or BDSM scene because you’re writing for the extreme erotic romance end, when it doesn’t feel in keeping with your characters or where they are in the story. BDSM in particular tends to involve a whole lifestyle so if it’s not already part of your plot then the hero isn’t likely to suddenly whip out the cable ties and nipple clamps.
In other words then, the only restrictions on your erotic scenes are those imposed by your characters and your story, and if you’ve explored your story people and plotted ahead, these will speak for themselves.
The list of things you absolutely can’t write in any category or genre is fairly obvious, but you’ll still find them stated clearly in all publisher’s guidelines. Which does make you wonder how many editors still receive manuscripts of this kind. These lists are usually along the lines of
No forced BDSM
No body waste fluids (although one or two mainstream erotica novels featuring ‘golden showers’ have recently hit the headlines).
No necrophilia (vampires and even possibly zombies are fine)
No bestiality (werewolves etc are fine, but not when they’re in animal form)
Of course, topics such as rape and incest do come up in commercial fiction, as well as some of the other nasties, but as harrowing scenes in crime and horror novels, not as erotic encounters intended to arouse. Thankfully I’m pretty sure that for the majority of us, this goes without saying.