I think all other achievements are self-explanatory, so I'll only write about the endings.

Important! Be sure to make a save before entering the portal!

When I finished the game, I reloaded the save I made right before I entered the portal and tried to figure out how to get the other two endings. Since after entering the portal, the game no longer offers a choice and I didn't figure out what to change before that, so I looked at the discussions to see if I could find any information.
Someone already asked the question how to achieve the different endings and the developer answered it:

"What you say when you talk to girls matters. If you choose mostly horny answers in dialogues you'll get the horny ending. :D Same with the other achievements."

I thought then that I lost a perfect game from my list since I wasn't going to play through the whole game again two more times. Then it occurred to me that if we talk to the girls again, the conversation will cycle through the 3x3 option again and again. I was wondering if this has any significance and the developer isn't such a sadistic bastard after all. :P Let's try it!

I reloaded the last save, right before the portal, and went through all the conversations with every girl again, but now consistently choosing a different type of answer than the ending I already had.
Then I entered the portal and yes, it works! I got that kind of ending. I reloaded the last save again and went through the conversations again choosing the third type of answers, entered the portal and got the last two achievements.

I don't think it's necessary to give perfectly uniform answers. I probably wouldn't have succeeded the first three times either. Only the majority must have a certain type of answer. If not in every single case, but probably mostly everyone can tell the difference between bored, average and horny answers.

Good luck!