You parents might know and understand that you're in a serious relationship, but still not want any sexual activity going on in their house. My mom is the same way ... She knows we're sexually active, and generally doesn't care, but she doesn't want it going on in HER house.
And that's just it ... It's THEIR house. You just might not be able to make them okay with this, and thats their choice.
Do you have any alone time at his house?
I'm not sure what else to tell you ...
We have a few threads on this that you might find helpful; http://www.scarleteen.com/forum/Forum13/HTML/000085.html http://www.scarleteen.com/forum/Forum13/HTML/000111.html http://www.scarleteen.com/forum/Forum13/HTML/000076.html