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I think that there's a lot of monsterfucker content that entirely ignores potential incompatibility in anatomy or tries to explain it, and while that's all well and good, I think there should be more love given to the realistic experience you'd likely have a lot as a monsterfucker in a world with monsters: physical attributes that are so disanalogous that it makes certain sex acts impossible without one or both partners being harmed.
It might seem harsh at first, but I think there's a lot of potential there. Not in the threat of harm itself, but in the methods of working around it.
Think about it:
A massive monster whose cock is too big for penetrative sex to be even a remote possibility. Try as you might, you can't even squeeze the head in. That's fine, though; they get extremely good at pleasuring you with their mouth, and you've found that with a creative mix licking their shaft and a well-placed vibrator, you can get them cumming so hard they'll forget they even cared about penetration in the first place.
A werewolf who can't top, not without instinctually clamping down on your shoulder with teeth long enough to reach bone, anyways. They'd never want to cause you harm, but you just aren't as sturdy as a werewolf bottoming would be, and you'd feel so good they know they couldn't help themself. It's hard to navigate, at first, but eventually, you learn that they make such an amazing bottom. It's so easy to get them collared, furry face pressed down against the bed as they claw desperately into the mattress, begging for your knot, even if you can't actually knot them (even better if you can, though).
A partner who is an eldritch monster. You can't even touch their physical matter, let alone fucker them. They certainly still have their fun with you, though. They mindfuck you; they seem to know the exact parts of your brain to stimulate to get you to feel like you're orgasming ten times over. Maybe even hypnosis stuff, if that's what you're into. And when they're done, they reach into your brain to comfort you, to make you feel all warm and held and to gently drift off to sleep. Even if they can't actually hold you.
Might add more to this later, definitely an idea I could expand upon more these were just my first examples.