To those asking about the game mechanics and such:
Can tentacles be avoided?: No, actually the very premise of the game is based on the fact that at the begining the MC is raped by demon-plant-tentacles and infected with an "evil seed". Also later on, there will be an unavoidable scene of demon-plant-tentacle rape.
Now, during the game in theory yes, if you are super careful and well equiped you can avoid tentacle rape, but as you will discover, being well equiped is not that easy on this game.
On the begining, you have little cash that you have to distribute between potions and armor.
As you can't level figthing enemies, you have to make a balance between fighting enemies to get gold and avoid them to save reources such as potions and probably the most important: TP (technique points?).
In this game you start with a short amount of TP, which is the base for you magical attacks and special skills.
You can use magic to use holy-based attacks, to heal and to repair your clothes.
You can use abilities to perform physical based special attacks.
The only way to recover TP is with items (1000 golds the "cheapest") or sleeping. And everytime you go to sleep is a day closer for you to become fully possesed by the evil seed. So be wary with those TPs.
About clothes destruction: Besides the MC main HP, your clothes have a certain degree of durability that will decrease when being hit by certain enemy attacks (claws, dissolving liquids), or even sometimes when walking through certain spots you will be attacked by tentacles that will rip your clothes. The more naked you are, the easier is for enemies to grab you and use "sex attacks". Also, if your clothes are fully ripped, your defense will be lowered so enemy attacks will cause more damage. You can repair your clothes with magic or with items.
About Sex attacks: Sex attacks will cause you to deplete your SP (sexual points?). If your SP reaches 0, the MC will have an orgasm during battle, and that will cause all your attacks to be weaker until you have calmed down.
So for defense purposes is always good to keep your SP high and your clothes in good shape. You can recover SP with items.
There are 2 ways to level up the MC:
Getting scenes of "sex": either getting raped my demons, monsters, bandits, prostitution or masturbation (this will occur automatically when going to sleep if you have high corruption). Everytime you have a sex scene, the "evil seed" gets stronger and your corruption grows. This will allow you to gain levels but also will take you closer to a bad ending of turning into demon plant.
Doing good deeds: Saving adventurers in problems, burying the dead, talking with the Sister in the church to get side-quests and helping the people on them, using items to clean graves, defeating mid-bossed or Bosses...
Just keep in mind that saving people often makes you fight strong enemies and specially on your first run they can be hard (given your limited resources and low level), which in turn can lead to your MC being abused and more corrupted.
In many scenarios, if you lose a fight (specially against bandits or human-type enemies), the MC will be abused multiple times and captured. Then you will play a scenario of trying to escape (completly naked, no items). If you are defeated during your escape attempt, you will be abused again multiple times, be returned to your cell, and will start your escape attempt all over again. Of course, after a number of times being abused, your level will increase and you'll be able to escape, with the cost of being more corrupted and closer to become a Demon-plant (bad ending).
So, how can you get a good ending?: Probably you won't make it on your first run. When you get a bad ending because you reached your time limit and became a demon, you will be given the option of new game+. You can keep your current level, items and money on a new game+, and it will be easier to beat the first part of the game while keeping the MC safe from nasty tentacles and villains.
So no need really to reload saves just to try to defeat a tough ENEMY. Just go along with it, watch all the cool CGIs and if you feel like you want to see the good ending, start a new game+
By the way, the password for the closed door near the end of the game is 010