I wrote two other responses to a total of probably 6 pages in rambling advice before realizing a simpler approach. First, I am also a beginner struggling with this but here is how I'm thinking about the problem:
Of course it's great to get something like this working (meaning full penetration, characters fucking, the usual) but it's really a waste of time to go through a million hoops and hurdles just for Jack and Jill to really bang out a loop of doggystyle. Especially when the satisfaction fades and I or you realize, this isn't a game, I just spent three weeks building not even a sex simulator, building a scene. I'm overexaggerating a bit but...
There has to be some sort of system. Sure you can get away with more if all of your characters are using the same skeleton but even then, different chest size or amount of body fat, we can easily come up with a full page of variables that will change the way the physics behaves. So we need to get the computer, the game engine specifically, to do that impossible amount of consideration for us -- by designing a more generalized system. We don't care what the 'specs' are for Becky, Sarah, Natalie, and Rachel, our code and game design should just incorporate whatever those 'specs' are and continue moving through the code. I am not familiar with Unity but I know a little bit about Unreal and your character has a capsule component specifically to keep the mesh triangulation from getting messed up (like when VaM characters explode and fly everywhere). I'm still learning and working on this too, maybe there's an easier way, I'm sure there is. But somehow that pointy male appendage cannot be restricted by the man's capsule or the woman's, which at the very least we could hard code as some sort of override but that's worthless if it's breaking the meshes, creating memory leaks, and crashing your game or hindering performance. So assuming we can negate the capsule component restrictions, I would consider the possibility of needing to protect the female mesh with some other limiting volume in the vagina so that in the very least you've still set a limitation on where the dick can go, therefore if it glitches and would ping-pong it's way to a bigger problem (again, think VaM), then at least now you've set additional limitations on the dick. Get the dick transform with a collision volume trigger and control the horizontal position of labia and vagina with the dick location and very stringent ranges so that things aren't opening when they shouldn't. To try and be more clear, write some sort of function as a part of the collision trigger that tells the left and right bones of the labia and vagina to change position away from the dick as a function of the shape and have the left and right sides 'mirror' each other while overall responding to the dick location. So if the dick goes up down left or right the opening changes in response. I've also seen (for Blender, but it would work in game engines) for body collision, you actually have the man collide with some small, invisible shape which you give the relevant properties to get the effect you need, for the woman's reactive motion to the impact you can parent things to the invisible shape, but it's probably easier to have a shape that has constraints/limits on it's position but isn't entirely static, collide with that shape to keep the character meshes from interfering, and map the woman's physical response and movement to the attributes you give this other shape (imagine a thrust, small cube get's hit and moves away but returns to it's position as the man is back again).
Rambling a little bit but I'm just as confused and struggling to figure all this out. There's always another way to do something in these engines and in code so even if I'm not wrong there's got to be an easier way but that's as far as I understand this when it comes to Unreal or maybe Unity. I know a bit in terms of Blender but it's entirely different. Hope this helps!
~pajamapanda
Edit: Some Youtube links that might help, researching this right now
You must be registered to see the links
^this would be a way to make the skin on skin deformation of the collision, just have the collision deformation only take place relative to the colliding character's position so that it reverses and you don't.. literally.. smash her..
You must be registered to see the links
^a Blender tutorial on visualizing soft body collisions of the type we're looking for, play around in the game engine to try and reproduce the effect
You must be registered to see the links
^I haven't watched this yet but there's probably some good info on how to organize some of the global concepts we'll need and also appears to cover the interrelation of hard and soft collisions (like the force of the sex impact constrained by the friction of her back on a bed or table simultaneously)
You must be registered to see the links
^breast physics simulation tutorial, there's always another way but again, closer to our specific goal
You must be registered to see the links
^more global/general approach to making a mesh 'soft' at all, use volumes or reference bones specifically and have them behave 'soft' on collision and go back to a more rigid but human state post-collision as a way of introducing a more natural constraint.. just an idea, probably not efficient but, idk
You must be registered to see the links
^facial expression morph targets for that sweet, sweet O face lol
there's more of course but idk, start there I'll keep looking
