Here's a quickie guide on some of the more easily missable items and scenes in the game thus far!

Oil- if you go talk to log near the start of the game, he rewards you with an oil pot, this can be used to have an easier fight with the Beastly Worshipper currently, the game  treats talking with Log vs Visceral-ing him as the same currently, but there are plans for alternate uses in later builds for the oil

Ectoplasm- For helping out the ghost dick, you can acquire some ectoplasm, this can be useful in the Worshipper fight, but will also have later uses in future builds

The Shabooby Grape- There's little incentive currently to not take this when you come across it, currently it only has two uses, one for duke and one for the blind woman, future builds may change this however

Iphone Apps- You can use two apps on the merchants phone before the Papa Gascoigne fight so long as you dont choose to jump right into grindr

Gascoigne- There's an easily missed scene with an emphasis on the dad's scent that comes with a warning about 'numbing mist' beforehand

Meeting the Vampire- If Duke chooses to open the coffin, down the road the vampire will remember him and give him and extra treat

Corrupt vs Forgive- The first of the mutually exclusive options thus far, Corrupting Alder offers some immediate spicy rewards and unlocks an exclusive bad end, but showing him some kindness can give Duke some interesting encounters down the road, like with certain shadows

Teach the Birds and the Bees- this I imagine is a common choice, but worth noting it does have a consequence free extra CG for choosing it over the others

The Hairpin- Risking it for the shiny when Duke senses an unseen presence unlocks a cute scene with the doll, for those of you who like being a nice person 

The Tonitris- currently only 25% missable,  but only due to a scripting error, you earn this for fighting the Dickbeast, which has its own set of spicy CGs that are easily missed, the game currently gives you this anyways if you happen to be wearing the choir garb, though this is unintended

The Drip- Aside from some minor dialogue difference, choosing to wear the choir garb when you've gained access to it currently unlocks an entire extra boss fight, with several custom art scenes and a chance to upgrade duke's power

The Bitches of Hemwick- Aside from seeing Duke's dick in a few extra sizes, fully losing against the bitches unlocks a hefty amount of cgs leading up to a bad end, 

That should be most of the easiest to miss content for now!