18 yo son wants to sleep with gf in our home / conflicts appears - help

Hello,Wife and I have a very conflictual situation incoming. 18 yo son, is wanting to sleep with his 5 month girlfriend in our house, and we have issues with that.Here is the detailed situation. Since last september, when he entered last year of high school (not in the US so I don't know the name of classes), his results went from acceptable to really-really bad. We also discovered he smokes cannabis and probably takes other products so we had conflicts about that, and enforced no smoking in house, and that we don't want to see him high at home.And he found a girlfriend. At first, she seemed nice and all, but we discovered the girl is in serious trouble at school, goes to illegal rave parties, introduced our son to alcohol drinking, drugs, etc... Since he met her he lies a lot about his whereabouts and we found out.3 weeks ago we went in vacation and he did not want to come with wife and I so we let him take care of the house and permitted that he invites some friends as long we found the house clean, and no complaints from anyone. We came back and discovered joints roaches, his room stinked (really, it was hard to breathe) dirt and smoke.The last few months, he asked to sleep with his girlfriend in our home. I did not want to. Wife was hesitating. The sex etc is not the issue, we had some serious talks about that.First problem : his girlfriends parents allow them at her home. He goes there like 3 times a week now. I am not happy with that because he now says "blalbal we can go to XX father place or to XX mother place and you two don't want me here"Even wife and I are arguing together because of this because she thinks maybe we could allow that.The reasons I am invoking are : - It is not a love hotel here. He does nearly nothing for things to run smooth at home, he does not pay the bills, he is a consumer and we don't want to have a little couple at home. (and home is a little house with no spare room) - If we allow it, I know he will ask every week, and that will drive me crazy - He has not any single argument. He refused the summer job I proposed to him. His results at school are terrible. We had to manage a lot of situations for him and he does not make any effortYesterday, I came back in the night from work, and discovered he was at home (wife was not aware of this !) with his girlfriend without asking me if it was ok and knowing that I did not allow it. This made me sick.I am to the point that if I listen to myself, I would kick him out of the house.Sorry it feels like a rant but I need some support. I don't know how to handle so much issues with a young adult who seems planless about his future. via /r/Parenting http://ift.tt/29qL4cj