I don't really understand Japanese and I block these games from internet so the live translation doesn't work so I might easily miss something, and I can't exactly recall every step properly so forgive me for any mistakes.
That being said, the progress was something like this:
Beat the enemy outside police station. Sleep at home, 1st door at right-side area from police station. Beat enemies outside of home, walk left towards police station. Enter sewer.
Make it to down-right sewer. Enter 2nd level, find the guy who gives you the key. Make your way to down-left of 2nd floor. Blow the obstacle with a canister, enter boss, beat boss. Had to collect some other semen too, dogs and rats I believe. Rats only drop it when you allow them to suck your teat, so you have to let them strip you and violate you. Once you have the stuff, go to the police chick. She'll send you to the last house on your home street to speak with the scientist. You had to sleep at some point here too.
I believe after the scientist you go to sleep and get a dream of the patient 0 again, the guy from first scene who entered the machine. Navigate your way to down-right (of level 1, I think) and blow up an obstacle with another canister, beat the boss inside.
Heroine wakes up. Can't recall if you had to chat with the police here or not but take the school uniform with you from your closet or buy one from the shop if you somehow misplaced it. Make your way to the right from your home street until you come up at a guardhouse. I can't recall the order of things here, you should get a scene of patient zero and his lady outside in the world infecting people but you should also get a scene of school girls at the guardhouse getting perved at by the guard. If you get the girl scene, switch to your school uniform and talk to the guy. He should let you inside the lab to tour it.
Inside the lab, talk to the employees. The right-most guy at the street level has the access card to the inner labs, get it from him and enter the labs at 2nd floor left side. Now you must navigate the same lab from different direction as with the patient zero dream sequence. Your goal is the same place where you'll face a boss here.
I believe after beating this boss you get out, get some sleep again and talk to the police again. Coming out you should see two men talking next to the station, ask them about the S&M club. Enter the club, get the acceptable outfit, wear it to get in further. Speak with the nympho lady, she'll get you her access card. Leave the club, go back to the lab and enter the red blast door on the upper floor of one of the levels. (You probably saw it by now on your previous scenes here and thought it looked important.)
Here you will face the end boss after navigating the level. I assume there's at least two endings because I fucked the boss to death in mine and got a pregnancy ending. One could assume that there's a "true ending" of you never getting fucked and therefore never ending pregnant, but who knows.
PS. If at any point the story doesn't progress like described here, try talking with the police chick or sleeping. I probably missed some of those points.
