Only off screen. It's a family show & the BBC doesn't want to embarrass parents.
To expand: the series was originally created as a children's programme, and although it has since become a true family show, the BBC still treats it as such in many areas. That said, not every show needs to be loaded with sex. Doctor Who is a bit of an oasis in which sex, while not ignored - it's a driving plot point behind "The Girl in the Fireplace" and "A Good Man Goes to War" - is not put front-and-centre. Some of the novels during the Virgin New Adventures era crossed the line a few times in the minds of some fans. But one of the reasons Torchwood was created was in order for darker and more sexual stories to be told which simply would not be accepted - by the BBC or by viewers - on Doctor Who.