If it's a stranger you're talking about you need planning and thinking (you know, someone you met a few days ago). With a partner, sure, you can do it in a second without any thinking. In the house with strangers, you really have to have a think before you copulate, no?
I've had sex with someone within half an hour of first meeting them. Again, you're making generalisations, which is precisely why organic things like sex should be allowed in the house, because obviously everyone's experiences differ, and the point of Big Brother is to observe people and learn about the ways in which people other than yourself interact.