Is it a Sin?

OK I Recently moved in with a best friend and his other friend not to long ago but his friend has a girlfriend who also gets invited in, all of a sudden after like a week my Best friends friend invites his girlfriend to live with us and my best friend agrees with him, now I'm am wanting to know if that is bad for me to live with them cause I'm a catholic and there not eventhough they know what I mean when I tell them that this is not right for a Girl to be living with 3 guys especially when my Religion is strict about certain things but they refuse to listen to me, My best fiend is a cool guy and he knows his **** and is god at doing stuff that other people can't and that is why I admire him a lot cause he has a better IQ then me but he does however have a bad side to him as well and lets just say its SEX even his friend to has the same idea about sex and he and she claims that they won't do it when I'm in the house but I refuse to believe them about that. So can someone tell me if I'm doing the right or wrong thing? I also knew what would go on when I moved out but Inviting friends over is OK but when Inviting someone to live with you is another story.