Partially because sexuality is so intertwined with everyday life, it's... tricky to reason about. But games mostly exaggerate some things while neglecting other things, so... 

Anyways, here's what I currently want to see in a game:

High arousal: health regen, magic regen, social contexts (not combat -- mostly -- see below), drains stamina, focus on specific "attractive" people.  Once high enough, out-of-focus options become excluded (perhaps conversation options grayed out)

Low arousal: When low enough: high stamina regen, sexual options become excluded (lead to health loss, magic loss, focus on "unattractive" people or things).

It's tough to think how to implement this, though. It's not so much that combat in games is super-realistic, but that we don't have anything similar for sexual contexts. What's would a "boss fuck" even look like? Aggro mechanics? Hit points? Aim? Combat games work because the underlying mechanics are so simple. Everything I want from a sex mod seems to want to tie into conversation trees, for some reason. And, implementing anything like that in a mod needs -- serializing the entire game's conversation trees (including decision variables and gating) to some manipulable format (Elminster's tes5dump, maybe) and then re-serializing the modified version back to a game mod file.

(Still... even just having better dress-up mechanics seems like a step in the right direction.)

But.. what do I think I would be trying to accomplish with all those mechanics?

Well... several things.

One thing is that there's meta information. Player installs mod means player has some expectations of mod. Player doesn't like play, and they uninstall the mod. And you can achieve finer control over this with mod configuration options. (And, because, real life arousal does tend to work like I described above -- when you're not aroused sex seems unpleasant and when you are sufficiently aroused absence of sex seems unpleasant -- having mod config options to put gameplay on roughly the same level where the player is seems critical. A problem here is that while almost no one in real life does combat, so we can get away with all sorts of silliness there without people blinking, everyone winds up with occasional sexual experiences.)

Beyond that, though, I am looking for the sexual equivalent of a "go do this simple task" [where you get ambushed and have to fight your way out]. This seems to want some foreshadowing, and plenty of other game mechanics, but also once you've gone past a certain point you're in the thing for duration. Bondage play can turn into solve a puzzle to escape, but beyond that how does it work? So... by tying sex to existing game mechanisms (health, stamina, ...) we start to get something beyond just a scene or a dead end in the game.

Meanwhile, skyrim seems like a perfect opportunity (if it didn't crash so much). There, magic is looked down on, and it's cold, which makes sex and magic great analogies for each other (well... with some revamps of the magic system). But it's important,  But I keep feeling that the mods people have done which forbid sex in towns are on the right track but... not quite right.

Here's my problem:  how do we make a good sex mod that deals with us not being able to adequately think about other parts of an arousal cycle than the parts we're currently experiencing??

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So.. combat: combat mechanics just don't work with sex mechanics unless you can enter a mindset where you want your partners dead. And if you want that, sex wind up being.... brief (and, I think, unsatisfying). I've seen mods that do this. They're probably great for the right person. But I am not the right person.

But, complementing combat? Using sex for combat recovery seems like a great fit.

But, also... just like there needs to be penalties (injury, you die, etc.) for mishandling combat, it seems like we should have problems with mishandled sex.  Losing gear is one variant there (but mediocre gear is often good enough and easy to obtain... or easy to make impossible to obtain -- and if its to be a good a game it needs to bridge the two possibilities: can't be impossible, can't be trivially avoidable.

Also, rpg style sex needs to have a progression, and since my favorite part is the setup, for me the game needs to have reasons (and perks) for setups (and escapes?). Happily ever after is great for real life and for the end of a book, but it's also game over. So... somehow the conflicts need to ramp up, also?

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Finally -- and this ties into using sex for combat recovery -- a way of introducing all sorts of quick sex scenes into a game without disrupting normal game mechanics would be dream sequences. These sequences can tie into a plot line by featuring recent (or maybe even foreshadowed) events, locations and/or people.  For this to work effectively, though, I think sleep has to become mandatory somehow. I do not like all the "real needs" mods mechanics -- they tend to not fit in with a the magic system, from my point of view -- but a gradual loss of max stamina and stamina recovery after being awake a long time (24 hours?) seems reasonable.