I personally wouldn't judge them for It, but I would also not be down for it.
I would feel awkward having sex with his parents in the next room LOL
I mean I know a guy who lived with his mother, because she had extreme mental health issues, and she needed the help around the house, and with taking her meds, meals, etc. I thought that was really sweet. and I also dated a guy who lived with his sister and mom. I got a long with them both really well, and would even hang out with his sister when I was over. I guess It all depends if his family is laidback and cool, and I felt like I could be myself. I have been living away from my parents since I was 17 so I'm not about that life.
I guess I feel like it could be really awkward and uncomfortable. I wouldn't want my boyfriends parents knowing everything that was going on in my love/sex life lol. Privacy is a must in a relationship for me.